- He doesn't age. In whassername's memory of the city being obliterated, he looked to be in his late teens or early twenties. Now, more than a decade later, he still does.
- He has computer-precise aim. He can change the trajectory of bullets by throwing pebbles at them, for crying out loud.
- He has some sort of deflector shield. Over and over again, he stands in the middle of a hail of gunfire, rocketfire, and twenty-foot-razor-boomerang fire, and when the dust clears, there's a Vash-shaped silhouette and everything behind him is obliterated. It's even in the opening credits.
- Machine empathy. Whenever a machine is going bonkers and gonna kill everybody, we suddenly cut away to another character, and when we see Vash again, the machine is either amazingly repaired, shut down, or fallen into pieces where it was standing -- and Vash is ambivalent about what happened. I'm guessing that the reason this is always off-screen is because it's too much of a giveaway if you actually see what happens.
I mentioned this theory to jamesbarrett (who's seen the whole series), and he seemed intrigued by it. But, as I told him I didn't want any spoilers, he wouldn't confirm or deny it.
I'll be interested to see, as time goes on, how far off base I am. ;)