They're legit. I flew Vueling from Barca to Granada and it was fine. I also encountered the credit card issue when trying to book online, so I just called them, spoke to some cute-sounding Spanish broad with good English, and it was good to go. They actually didn't charge my card for the ticket, but locked in the price for me and I just had to show up and pay for it before 2 hours until take off, which I did. I've never had another airline do that, and in retrospect it's probably not the best move, but it worked fine for me.
May is a pretty cheap time of year to travel in Spain as it's right in between their two busiest periods, so don't be weirded out by the low prices. All is good. But I didn't find the AVE train to be a very good option, both in terms of price and convenience.
May is a pretty cheap time of year to travel in Spain as it's right in between their two busiest periods, so don't be weirded out by the low prices. All is good. But I didn't find the AVE train to be a very good option, both in terms of price and convenience.