i think its where you also can buy clone characters/costumes a la dr mario for mario etc.
also if youve ever played the newer mortal kombat games, that vault has like sections, videos, characters, and sometimes they dont tell you what they are and you just have to pay for them and hope you get something good. MAYBE something like that but i doubt it.
but wait! There will still be way more than that. Sakurai hasn't told us everything. Brawl has so much stuff on it........i don't see how it all fits on one disc lol. I say there'd be at least 3-4 more things in the vault.
I think that Home run contest, Target Smash, ect. type things will be when you go to Solo mode and those option will be in the Solo Section. I think the Vault will have Coin Launcher, trophies, stickers, picture memory holder thing, replays, Stats.