The problem is that even the characters are broken in various ways, which makes even making a game using only the characters for Vita a bit of a challenge. I'll try getting the characters fixed with Dolmexica first and let you know when I have something that works on Vita.
It is unfortunately unlikely for me to try something like that, as I always try to make sure the Dolmexica mods I make also run on the Dreamcast. And in my experience, anything from 3D games and from after the Dreamcast's lifespan does not fit in the Dreamcast's limited memory.
I am still fixing Dolmexica so it works with the Street Fighter VS Tekken mod, and the characters there seem to fit into Vita memory at first glance. Maybe someone has already made Street Fighter IV / Tekken mods for Mugen? If those don't work with Dolmexica, I would also take a look at those, but I wouldn't proactively make new ones.
lots of cross over games are already made for mugen over the years but doesn't work with dolmexica. I tried to use the assets to dolmexica but it crashes in PS Vita. I don't have a dreamcast but I can try it using flycast. I have tried your dreamcast versions it works on flycast but no one have mod a mugen game for dolmexica or dreamcast with tekken or streetfighter theme or even a cross over.