I have just put up a video on YouTube that may give you some useful tips about helping bees defend their colony against wasps (yellowjackets) http://www.youtube.com/watch?v=7JqFo5lzCPY
I have never heard of this being a problem in the US, if you maintain strong colonies. The only time I have seen yellowjackets picking on a beehive is when it was almost completely dead and eaten up by wax moth. They may have been feeding on honey for a sweet taste, but I believe that they were actually going after the brood. Being carniverous.
"Dude! Don't over think this! We're just cookin' hot dogs here...". Mark Berninghausen
Here on the west coast, southern california we do have a major problem eith yellow jackets.
We put out the pheromone traps and had to empty them every 2-3 days. The population of the yellow jackets is much lower this year.