To the east of where Magus Castle was, on the site of present-day Medina, stands a small castle. Underground Waterway plays here, probably for the creepy effect. Vinnegar himself stands atop a flight of stairs in the entryway.

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

Vinnegar has attached a title to his name. It's the same one Magus is using as his name, but with an intensifier added. For consistency, I'm using "Great Magus", but much like before, it's closer to "Great Demon King".

Vinnegar: ...what are you saying!?
The likes of you, who would abandon, to
kiss up to humans, 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 at this, it's
everyone, it sure has been a while.
Big sis does not like stubborn
little boys and girls.

Mayonnay: Oh my, it's Magus-sama.
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,
Great Magus Vinnegar-sama.
Leave dealing with these
naughty children to me.

He retreats again.

Battle time, against "Super Mayonnay". The fight is nothing special, though, and you can't even steal anything.

Mayonnay: Kiiii! 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. Ooh.

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

The battle music starts, and 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. 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 that looks like the one where he summoned Mayonnay.

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

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, against "Super Soysaw". But the fight's nothing special, and you 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 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 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, and partly 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 a skill that resembles Crono and Lucca's Fire Sword. All three of them counter hits on Super Mayonnay with 悪インパルス (Bad Impulse), a fire attack similar to Crono, Marle, and Frog's Arc Impulse, but dumber looking. All three of them counter hits on Great Vinnegar with ミックスデルタ (Mix Delta), which is essentially the same as the Crono+Marle+Lucca skill with the same name. Attacking all of them triggers all three counterattacks, so don't. Since most 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. Also, I hope you brought Ayla with you, since you can actually steal all three of the items they mentioned. See the items page for details.

Very minor wordplay here. そんなバカな (sonna baka na) is fairly common 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, and...

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

Vinnegar: Magus-sama....
Are you bent on beating me?
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 Demon's 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, you can steal Vinnegar Underwear from him if you didn't earlier, or even if you did. Attacking the switch ends the battle, but the hole opens up beneath the party this time.

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, a fleeting 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 Magus can use.

With the fall of Vinnegar 400 years earlier, things have changed a lot here. The little goblins now run the town, and they're peaceful and friendly. 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 or anything, really!
........sob, sob.
You don't have to pick on me like that....

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