They were just starting to come out when I was up there last week (7/01-7/06) and they weren't bad at all, worst was 10 or so in one camp one evening buzzing around the food. They were slow and easy to extinguish. Cooler temps the last two days of our trip may have helped to keep the numbers down.
As far as traps go, I've had real good luck with these in my yard where we seem to have hundreds of paper wasps every summer. While a paper wasp is not a yellow jacket, I assume the trap will work as well.
RESCUE!® - WHY Trap for Wasps, Hornets & Yellowjackets