I agree with RAB - there's no substitute for a really strong colony. If you've got several modest-sized colonies, unite 'em... and be bold when it comes to reducing entrance size. With OMFs, they don't need any ventilation from the entrance, only enough space for one or two bees side by side. It doesn't matter if they have to queue a bit. A tube would probably help too, but I still think entrance area is the most important factor.
Loads of wasps around isn't a problem UNLESS they are actually getting into the hive. Blatting a few bees at the entrance and/or making off with dead or dying bees isn't really an issue for colony survival... getting inside and hoovering out all the frames definitely is.
If there's a lot of hives full of brood and honey and the wasps can get in, I find that no kind of wasp trap will really work - you just can't compete with a strong-smelling boxful of delicious goodies. You can sometimes put them off at the entrance though: I discovered by accident that a mirror next to the entrance seems to confuse wasps without preventing bees from getting in. I think maybe the wasps rely more on sight, the bees more on smell. I notice that the wasps try to attack their reflections sometimes!