See, here's what I realized- there's this one episode of Trigun where Vash is all scruffy and long-haired, and I realized Hey, he looks exactly like Hohenheim! I screencapped it. See?
My theory is that Vash (and Knives as well, seeing as they're twins) are Homunculi spawned (once again) by Hohenheim. He really gets around, doesn't he? The theory is supported by Vash's age (130-ish) his perpetual youth, the fact that he and his brother grew from babies to around ten in one year, and that Vash can pretty much survive anything.
Of course, to make this work a hell of a lot of canon would have to be twisted, but hey! It's a crack theory! It doesn't have to make sense! :D It would sure make an interesting fic, though.
Or, if you don't want to bother with the crack-theory, here's some fanart of Ed the stampede. :)
The $$60 billion shrimp.