Flying into BCN, onto Naples

Our experience has been that the open jaw or multi city trips are rarely more expensive. Please post you exact dates of travels and flight cities. While few would recommend what you are suggesting, we have done it several times. But we allow at least five hours and maybe six. And it has been close a couple of times and one time it failed when the original flight from the US was cancelled. Took three days to get a seat out of the US. You can try but it does have risk. If you miss the connecting flight, what is plan B?

Does the carry on conform to both Air Canada and Vueling requirements?

You must be at the boarding gate 40 minutes before the departure of the flight, and the boarding gate closes 20 minutes before the scheduled departure time.

Found this quote on numerous sites, (but could not find the wording on the Vueling site). You may want to check the accuracy with Vueling as you may have less time than it looks between flights.

Your plan needs (near) perfect execution to be successful.

As Frank noted you should explore multi-city aka open jaw ticketing NOT one way tickets.

You should not be looking at two one-way flights. You need a "multi-city" ticket that takes you into Naples (or at least Rome if Naples is a lot more expensive) and home from Barcelona. Just choose "Multi-city" rather than "one-way" or "roundtrip" when you begin your flight search.

Are you going to spend any time in Rome or are you going directly to Naples after landing?

Because there are flights from BCN to NAP on Ryanair/Vueling/Easyjet. Sometimes there's a 1030 on Ryanair, which would give you a little more breathing room