I don't know for sure, but the rollout of Vueling codeshares seems to be taking a long while. The new NCL-BCN services are now available on BA.com under a BA codeshare but it's not been up for very long. I tried CWL too, my usual way of doing this is to do a dummy booking on VY to ensure it's a direct service on the date in question, then work backwards to BA.com. No dice.

Having said that, unless you're after Eligible Flights you're probably better off buying direct from VY. The BA price is above the Optima tariff, which is the tariff that allows you to get Avios via IB+. If you were looking to connect in BCN to another location in Spain, I don't think this would work anyway on BA.com