It would make sense for it to branded Vueling as it will be relying on its short-haul feed at BCN. This would also be consistent with IAG's approach of having brand consistency (for the most part at least) at hubs. Hence why IAG pushed ahead with Iberia Express at MAD in spite of opposition from the unions, rather than letting Vueling take over at Madrid.
But it won't be operated by Vueling. It had a very bad summer and IAG has curtailed its growth plans for the next year whilst it gets its operation back on track.
I can see a new airline with a new AOC linked to the Vueling brand with perhaps EI or IB wet leasing aircraft in its first year.