It's actually quite simple. Don't ever expect to use your Avios on Vueling award flights via Iberia Plus.

There is no set number of seats available as awards. It's simply based on airfare. If the airfare is cheap enough, you might see a Vueling flight pop up. Except it would give you a ridiculously low value for your Avios.

Even when you include the cost of checked luggage and seat selection (back rows only), you'd still be better off looking at the paid fares direct from Vueling or the various codeshares. Vueling awards are totally non-refundable, non-changeable so that standard advantage of awards isn't there either.

P.S. If you have a few Avios you want to get rid of and need a Vueling flight, you can also open a Vueling Club account and partially offset the fare by spending Avios, again at a poor rate...