Refund.me have two lawyers/legal experts
in "Espania"[sic] and seem to indicate that Spain
is covered. I've not used them so I can't make a personal recommendation.
However, Vueling's letters so far have been so error filled and inaccurate, that I would persist with dispassionate, short letters referencing the regulation and showing that their interpretation is wrong, and spelling out the Articles that show that compensation is due. They will probably stop communicating as their next tactic, but keep pressing, no need to to get an agent on board just yet (who will take a cut from your compensation).