As for BA, Vueling taking over LHR short-haul will never happen. Transfer passengers will expect to transfer on to BA and Club passengers will expect a short-haul business class product.
Nothing to stop Vuelling aircraft being re-branded with a BA product, staying on the Spanish registry and operating ex-LHR as a BA code-share? 'Welcome on board your British Airways flight operated today by Vuelling'. Think Cityflyer and OpenSkies. The cost savings are to big to ignore. Captains on 3,000 Euro and First Officers on 800 Euro a month.
What *might* happen is Vueling setting up a base at LGW to go head to head on short-haul routes that Easyjet has largely seen BA off with BA doing long-haul and holiday short-haul routes to the Canary Islands etc.
They already operate LGW-BCN so how long before a small base is established is anyones guess.