I am in another spanish airline but from what I heard, at the selection day the guys clearly says it is a temporary contract with no guaranty to be taken back on the next summer season and he also advice that if you have a family to feed or mortgage to pay back then you'd better stand up and go home before even take the tests because although people can see a "nice" vueling shiny airbus, they will work hard, long duties and very bad roster, but the worse comes to very very bad salary that makes life very difficult. Barcelona is not a cheap place anymore, lots of russian, chineese and other people with cash that make good business and renting a decent flat for less than 1000 euros (65% of vueling cadets salary in summer) is just impossible. The only option is to shsre a flat with another 3 poor people, like erasmus student do, or go to the outside the city in gipsy places like Badalona or Casteldefells.
Really it is a worse deal than Wizzair or Ryanair, except BCN or FCO base better than polish ****ty places, Vueling is not a good deal at all, but this is my own opinion.
By the way, last year apparently many people failed their test. If what I heard is true there will be other "convocatoria" dates in January and february. At least.