Sherlock77, one of my interests is collecting toy soldiers and when I would go to flea markets or shows I always tried to get there as early as possible and priority one was making a fast sweep of the entire place to look for rare finds or bargains. Once that was done I would then circulate more slowly and chat with dealers, some of which I got to know. Maybe you could make a first fast pass getting your pictures and then stop to talk with folks later.
This strategy doesn't work all that well for cruise nights, which only last for about two hours and are far more fluid than a daytime weekend car show... People come and go with their cars far more frequently... And because I'm still in the world of work I generally end up arriving after it starts, which means I don't know what I missed (ie. a car that was there earlier, and left before I arrived)
It's a love/hate thing, I do like how fluid these evenings are, but it's very challenging to get a good photo...