Welcome to Flyertalk and welcome to the BA forum.

According to the note on Vueling.com the "combine Avios" screens have been switched off temporarily - and that means for several months. This used to be possible via Avios.com but they broke the auto-move process in some coding changes and then went to using a chat system where it could then happen. Then they decommissioned that Avios platform in preparation for direct access. Aer Lingus was in a similar place, and in that case of Aer Lingus it has been brought back. Vueling seems next in the queue. I can see my own Vueling account and Avios therein, but there isn't a Combine option on the screens at the moment. Knowing Vueling they probably want their chatbot to steer the process, but at the moment the bot just refers back to the Avios page