If they want the brand AND want to base out of the UK then, for all aircraft in excess of 100 seats, they must be flown by BA mainline flight crew. That was legally ratified during the Openskies dispute and even IAG couldn't get round it.
IF they want to brand it BA.
Vueling has nowhere near the marketing impact, brand wise, as BA. It would be a big balls decision to make considering that the prime aim of the BA shorthaul network is as a feeder to the LH network with continuity of brand/product etc.