So if VS can fill a decent number of those premium seats, and I imagine that was a factor in the decision to switch to VS, then it should certainly help yield, although I've no idea what the situation is on cost bases.
Virgin, like Laker before them, do not pay cabin crew....well. Freddie Laker once said they were ten a penny and easily replaceable, unlike pilots. Their US counterparts are more "senior" so if they swap a DL operated B763 for a VS operated A333, their is a good cost saving so long as you can still fill the seats.