In standard Spanish in Spain, "b" and "v" are pronounced the same way, which is /b/. It is true that in some Catalan dialects they are pronounced differently, but Catalan is another language.
Wikipedia in English gives you the Spanish pronunciation:
[ˈbwɛliŋ];
So there you go, problem solved.
English versions are numerous, but that is the official Spanish pronunciation.
I don't know whether it has been said already, but their name comes from Spanish "vuelo" (flight) and the English suffix -ing. Their marketing in the early years consisted in a mixture of Spanish and English words, later with other languages. That gave them a "fresh" look and attracted loads of young people. They had a cloud as a symbol as well. This all went with the merge with Clickair, which was a bit sad.
EDIT (answer to previous post): for most Spanish people, "Easyjet" is as unpronounceable as "Vueling" for British Speakers, mainly because they don't have some of the sounds, as the "j" (pronounced differently in Spanish). What about Lufthansa? Wizzair? British Airways? Monarch? the last two especially are hard work for some people in Spain.
I can't see that as a problem. The name is catchy. "Volotea" for instance must be surely harder to pronounce well for British people.