To the east of where Magus Castle was, on the site of Medina in the Modern Age, stands a small castle. A version of "Underground Waterway" that lacks the watery effects plays here, probably for the eerie feel. Vinnegar himself stands atop a flight of stairs in the entryway.

Vinnegar: Welllllcome!
This is the Grand Magus Vinnegar's...
　　　　　　　　!?

Vinnegar has attached a title to his name. It's the same one the Magus uses in place of a name, but with an intensifier added. For consistency, I'm using "Grand Magus", but much like before, it's arguably closer to "Grand Demon King".

Vinnegar looks shocked and utters the following garbled and apparently meaningless noise, adapted to something similarly meaningless:

Vinnegar: Gblarg!!

After checking into it more thoroughly, I've found that in this context, いい身分 (ii mibun) amounts to a snarky comment used when someone has, in the speaker's opinion, managed to get into an unfairly comfortable situation, one that he never earned and certainly does not deserve.

Vinnegar: ...what are you saying!?
The likes of you, who kiss up to humans
and abandon our battle to build a world of
Demons, are no more king to us!

Vinnegar retreats into the castle, but stops in the next room.

Vinnegar: Y-you obstinate bastards!
Fine, then...

The room dims and Mayonnay appears, as giddy as ever.

Mayonnay: Here!
You called for me?

Mayonnay: Oh my, look who we have
here. It sure has been a while.
Big sis does not like stubborn
little boys and girls.

Mayonnay: Oh my, it's the great Magus.
I wonder how you have the nerve to
come here. Big sis will punish the
traitor who would side with humans.

Mayonnay: Now, stand back,
O Grand Magus Vinnegar.
Leave dealing with these
naughty children to me.

He retreats again.

Battle time. Super Mayonnay (Flea Plus), monster ID 187, is nothing special, though, and Ayla can't even steal anything.

Mayonnay: Urgh! How irritating!!
You'll pay for this!

Mayonnay retreats too. The next room has chains, conveyor belts, and pits on the sides of the room. Vinnegar stands on a ledge above, next to a crank.

Vinnegar: Niys too mee chew!
Come forth, my servants!
Make mincemeat of these bastards
who are trying to take back the treasure
I carried off from Magus Castle!!

Try saying the first line out loud. He's saying "Nice to meet you" in very poor English in the original, so I tried to preserve the awfulness while making it somewhat readable. And it's treasure from Magus Castle, not just any treasure. Oooh...

He starts turning the crank, and monsters come up the chains.

Possibly a reference to a variety show of that name.

The battle music starts, everyone pulls out their weapons, and the monsters look menacing for a few seconds, but they just fall into the pits on the other side of the conveyor belts. Oops. Vinnegar's Manor may have similar architecture to Magus Castle, but apparently less care was put into making sure things actually work the way they're supposed to. The music slows to a halt, and everyone looks perplexed.

Vinnegar: Vi...
Vinnegar PINCH!!

He fades out of the room, and the normal music resumes. A little further in, Vinnegar stands at the far side of a room much like the one where he summoned Mayonnay.

Vinnegar: Y-you still mean to follow me?
Fine, then...

The room dims and Soysaw appears, with rather more dignity.

Vinnegar retreats, again.

Soysaw: I suffered defeat to you the other day,
but it won't happen a second time.

Soysaw: I am reluctant to cross swords with
my former liege and one whom I revered...
but if this, too, is fate, I have little choice!

Bad... NA... translation... brain... hurts...

Battle time. Super Soysaw (Super Slash), monster ID 186, is nothing special either, and Ayla still can't even steal anything.

Soysaw retreats too. In the next room, Vinnegar stands by another crank, and a blade on a chain dangles by a treasure chest.

He turns the crank, and the blade moves up and down.

Vinnegar: Hyo, hyo!
You fell for it!!

Vinnegar speeds up the cranking, and the blade goes nuts, reducing everyone to 1 hit point. He laughs and disappears.

Vinnegar: Um, hello, the treasure box?
Aren't you going to take it?
I'm sure it's got something good in it!

A Jarry (one of the little goblin monsters) walks in and sees the chest.

It walks right into the blade, gets chopped up, and dies.

Vinnegar: Vi...
Vinnegar SHOCK!!

Vinnegar disappears.

The chest has the same contents either way, so why not wait until it's safe to take? Vinnegar, Mayonnay, and Soysaw await the party in the next room.

Vinnegar: Hu, hu, hu, third time's a charm.
This time, we won't be beaten so easily.

Vinnegar: Vinnegar Underpants!

Vinnegar: With these sure-kill items
we have equipped, we'll beat you
bastards to a bloody lifeless
unrecognizable pulp!

I'm taking some liberties with what he intends to do to them, but that's basically his meaning.

It's time for another battle! But this time it's actually challenging, partly because all three are here, but mostly because they have special abilities and counterattacks. They even get the special alternate boss music.

With help from Super Mayonnay, Super Soysaw counters, whenever hit, using an unnamed skill that resembles Crono and Lucca's Fire Sword (there's something in the combat text about a Heartfire Sword that might be for this, but it never displays in-game). All three of them counter hits against Super Mayonnay with...

...a Fire attack similar to Crono, Marle, and Frog's Arc Impulse, if dumber looking. Also note the phonetic similarity between the usual アーク (AAKU, ark) and the 悪 (aku, bad/evil) here. I sort of want to call it "Dark Impulse", but that would suggest it's Dark-elemental, which it isn't.

All three of them counter hits on Great Vinnegar with...

...which is essentially the same as the Crono+Marle+Lucca skill with the same name. Attacking all three at once triggers all three counterattacks at once, so don't. Since many of these attacks are Fire-based, equip some Fire protection. Most of their power comes from the counterattacks, and they can only use them when all involved remain in the fight. Kill Super Soysaw first to make things easier. Super Mayonnay will run away shortly if Super Soysaw falls, and Great Vinnegar is predictably helpless when alone.

Alternately, if you have Dark absorb or enough magic defense to handle their counters, taking out just Vinnegar wins the battle automatically.

Also, I hope you brought Ayla with you, since you can actually steal all three of the items they mentioned, but be warned that this provokes counterattacks. Although the Vinnegar Underpants are predictably worthless despite their high defense, the Soysaw Katana 2 is a fairly good sword for Crono, and the Mayonnay Bra provides the best bonus to magic defense of any accessory. See the items page for details.

Very minor wordplay here. そんなバカな (sonna BAKA na) occurs fairly often as a "no way!" expression, and あほう (ahou) and ばか (baka) both mean (approximately) idiotic, so substitute アホ (AHO) for baka... Or it might just be a regional variant.

Vinnegar retreats yet again. The party finds him upstairs, with three switches on the wall behind him.

Vinnegar: Great Magus...
Are you bent on taking me down?
Was our dream of fighting together
to create a world of Demons a lie!?

Magus: Hmph...
I just wanted power.
Perhaps you can hear it too.
The sound of the black wind...

Vinnegar: I won't lose! What would
become of the Demons' future if I lost!?
I simply can't afford to lose!

Vinnegar (now just Vinnegar, not even Great Vinnegar) resorts to his trick of hiding in an impenetrable barrier. Incidentally, Ayla can steal Vinnegar Underpants from him if she didn't earlier, or even if she did. Attacking the switch ends the battle, but this time the hole opens up beneath the party.

This is monster ID 167 again, just like when you face him earlier in Magus Castle. The only thing keeping you from stealing Vinnegar Underpants then is that Ayla can't be in the party at that point.

Vinnegar: Hyo, hyo!
You fell for it!!

However, they just land, unharmed, in the room below, and can come right back up.

Vinnegar: Useless, useless, USELESS!
You bastards can't beat me!

The boss music starts up, but before anyone can do anything, a cat walks up and hits one of the switches.

A hole opens under Vinnegar, and he falls and goes splat.

Lucca: ......
What was up with him?

Ayla: What he want do?
Ayla not understand.

Magus: What's past is past.
Everything, an ephemeral dream...

Don't leave just yet... remember that treasure Vinnegar mentioned? In the room where he trapped the chest, go down from the right side of the stairs. There's a passage through the wall to a small room with a Magic Capsule and three chests. The chests contain the Mantle of Despair, the Helmet of Despair, and the Scythe of Despair, very powerful equipment that only the Magus can use.

With the fall of Vinnegar 400 years earlier, things have changed a lot here. The little goblins now run the town, they're peaceful and friendly, and "Merry Specchio" plays. The merchant, far from overcharging, actually gives a 20% discount compared to other shops. Not only that, but all the bigger, more warlike Demons are doing all the menial tasks. The most amusing change is in the chief's house. A blue Jarry sits in the chief's couch...

...there's a human here...

...and even Vinnegar's descendant is on cleaning duty.

Vinnegar VIII: Yes!
I am cleaning with the utmost effort.
I am not slacking off at all, I swear!
........*sob*, *sob*.
You don't have to pick on me like that...

Well, that's the end of that sidequest. Rewards: Vinnegar Underpants (steal up to 2), Soysaw Katana 2 (steal), Mayonnay Bra (steal), Magic Capsule, Mantle of Despair, Helmet of Despair, Scythe of Despair, various other treasure, 20% off item prices in Medina, and getting rid of that hideous statue in the Medina village square.

With the Magus having vanished after his failed summoning of Lavos, and Vinnegar utterly stripped of any credibility or dignity he may have had, the warmongers must have looked like fools. The resulting impact on prevailing attitudes and relations with the humans has had a drastic effect on the situation in the Modern Age.