Re: Yellow Jackets casually coming and going
3/4 inch opening is small enough for bees to guard, but also the entrance should be in the middle, or right next to the bee cluster. in a weak hive if the entrance is off to the side away from the bee cluster the bees are less inclined, or able, to guard it.
Secondly if the yellow jackets have found the syrup they will keep trying to get it and a syrup feeder is impossible for a weak hive to guard. If the bees need feeding, try to give them a solid shot of syrup mixed as thick as you can, to get the job done quickly & remove the feeder. Mix it thick cos it's the sugar you are feeding them, not water.
And another factor is varroa mites, the more mites a hive has the more prone to wasp attack. Cos the bees throw out affected larvae which is a draw for wasps plus the bees are generally debilitated even though it may not appear that way at the entrance. In my yards if I see wasps focussing on one particular hive and entering it, it normally turns out that hive has a higher than acceptable varroa level.
44 years, been commercial, outfits up to 4000 hives, now 120 hives and 200 nucs as a hobby, selling bees. T (mostly).