They have designated groups, like Ghostbusters, Anime/Video Games, Star Wars, etc. So they'll manage to fit you somewhere regardless of your costume. Not just /anyone/ can walk in. You have to sign up and have a little ribbon. Some people hopped in last year, but it may be more enforced for this one.
As for vehicles, I have no idea. Most of the celebs or whatever that walked didn't walk...but...rode. xD And then the Ghostbusters had their car and there were some bikers. You have to ask the lady/dude organizing the parade this year to see.
You can find most of ALL this information on the Dragon*Con website.
Dragon*Con '11- Sansa Stark, Little Sister, Belldandy, Rainbow Dash