This will be my first year going but a friend of mine who goes every year with his schwinn consortium tells it like this:
Go early, 5-6, its first come-first serve and $20.00 to park your car in the field (supposedly you get a T-shirt too, this is going to be the 20th year.)
Some people get there early and park on the main road-JFK Drive, they have front row seats, good parking and save themselves $20.00
I was told no vendors just a picnic with cool cars, I don't know if selling is allowed there or not.
It is in Speedway meadows in GG Park, which is Just north of the Highway 1 overpass which goes through the park from 19th Avenue.
If you make it look for the raggedy Plymouth
Hers the link: http://www.jimmyspicnic.com/
Oh and cars have to be pre-1972