Thanks to all for your insights. The more I research, the more questions I have.

I will be traveling with two bags, and I'm concerned whether they'll qualify as carry-on luggage. The first is a Timbuk2 20 inch roller bag (size small), which is going to be very close to the 55 x 40 x 22 cm allowance for hand baggage, after accounting for wheels and handles. The second bag is a backpack whose measurements are 20 x 13.5 x 9 inches (51 x 35 x 23cm). Vueling's guidelines for the personal item aren't specific. The backpack fits under the seat, if I don't stuff it full. I usually don't check bags, but will my two bags exceed what Vueling will allow as hand luggage? My choice of seating will depend on whether I can carry everything onboard or not.