Vasectomy shouldn't change the way it behaves at all, castration does. Castrated hobs would probably work better than entire or vasectomised hobs come spring time as their minds aren't on knocking boots.
Can you lock a pair in the box? You won't be able to easily rear youngsters with the adults on rations and you won't get the others back in if you feed a lot.
I've found dogs like pigeon eggs too ?
I would, stick them on half rations for a couple of days first too so that they are watching the feeder like a hawk and whistle or call however you like at feeding time until they respond to it.
Get em on wheat mate, maybe pigeon mix on a weekend or something. When they are on wheat they will fly slower and kit better.
The cleaned wheat from the shops is only a tad dearer than dusty farm wheat.
I think it would be better with a silenced pcp rather than a springer due the boing as they're fired possibly. Overcast nights are meant to be decent too as it's easier to see them in the scope ?