Lee,
When I first saw the robbing activity I reduced the entrance to about 3/8" by 3/4". There were hundreds of dead yellow jackets on the ground in front of the hive. I was tied up for a few days and when I checked back the YJs were mobbing all the entrances. So I stapled makeshift robber screens over the entrances. That seemed to stop them. All this time it was too cold to be open the hive and mucking about-mid 50s. When we had a warm day I went in and found many dead bees but no queen. I found a dead queen on the bottom board of one hive. At most no hive had more than a dozen bees.
I suspect that when the pressure got really bad the living queens and bees just absconded. When I put out the robber screens I also placed soda bottle YJ traps with syrup for bait. I guesstimate I killed about 10,000 of the little suckers, but it was too late. Next year, I will put out traps mid-summer with rotton meat for bait. I've done this in years past and it works well. like your post indicates at the end of broodrearing season they go nuts for anything sweet, but in summer time meat will draw them in better than anything.
Only glimmer of good news in this-when I realized my hives were dead, I left them out and they were soon covered with ferals robbing them out-there wasn't much. I lined out two directions they were coming and going from. Next year, I will line them out and maybe rob em back.
I have made a friend in the beekeeper's class I've been taking and he has some good bees about 3 hours from here to help me get back in it again.