It's really not that uncommon to buy an extra seat if the passenger is larger than average or they're travelling with a large valuable piece of hand luggage like a musical instrument. On most airlines, the extra seat costs the same as the fare paid but without the taxes and fees added. On vueling.com for a cash booking, you just tick the "extra seat" box and it charges you 2x the fare but you only fill in one passenger's details.

So I suspect they'll know what I'm on about, whether or not they know how to do it for a redemption.