Take it up further?

Iberia can change the terms, conditions and benefits of Iberia Plus at any time. So you can get a lawyer and take Iberia to court over failing to deliver promised benefits, which will prompt Iberia to update the website and stop promising benefits on Vueling flights. I can't see any solution that involves IB informing all European out-stations that [MENTION=645239]mrshyt[/MENTION] is entitled to lounge access on Vueling flights.

P.S. I share your frustration at being based near BCN and having few short-haul options apart from Vueling. I also think it is a horrible idea to be putting BA/IB codes on a low-cost carrier.