Vueling's load was 48% for the month TA, which i believe is half decent for the time of year i.e, being april, and considering it is the first month of operations, where the airline has had to gain brand identity and all that. It will probably be seasonal on all services to be honest though, BCN, PMI and ALC.
Alex Cruz said initially that they intend to stay at CWL not for just one season, that being a quote from himself. So, Vueling will be back for Summer 2013 i believe. Also, they chose CWL over both SOU and EDI for their first UK-BCN flight (discarding the EDI ones back in 2007). They must have had confidence from somewhere. I will put a bet on vueling achieving good loads throughout the Summer season i.e being May, June, July and August. PMI and ALC are supposedly looking good, with BCN holding up well also. So to put it your way is a tad harsh.