http://magic.wizards.com/en/events/coverage/vintagechamp15/final-standings-2015-08-23looks like the magic number is 484.
it also says, "NOTE: FINAL STANDINGS REFLECT THE NUMBER OF PLAYERS PARTICIPATING IN THE EVENT, NOT THE NUMBER OF PREREGISTERED PLAYERS."
there were also people there that just registered for the playmats and didn't play at all. well, a guy did register 60 mountains next to me and put himself in the budget category so I can only assume... so maybe that's where the 460 number came from.