Have you ever tried going to the Iberia check-in desk at the airports where you have problems? My suggestion is to check-in on-line or at the Vueling desk, and then proceed to the Iberia desk to claim your Iberia Plus Gold benefits (together with your boarding pass) to get fast-track and lounge access. Obviously this is only possible if there is an overlap between your Vueling flight and and Iberia Group flight.

If you fly often with VY then BAEC Gold or any other ffq is completely useless. You have no benefits with BAEC Gold while flying Vueling - in fact, when you buy a VY flight with BA code, you only gain Avios (no Tier Points). So if you aim to get a tier on any program, unless you want the Vueling own program, IB+ is really your only option.