I think JC is right and that it varies. One of my brothers in law lived in Mexico for10 years and speaks Spanish quite well, though he has kinda danced around the subjunctive. I'm just a student but I don't recommend doing that. Anyway, he has said that when inquiring about spelling folks will ask: ¿"beh" burro o "beh" vaca to clarify. Also "beh " grande or "beh" pequeño. I have been watching some free learning videos with speakers from Spain and I hear a "b" particularly with a word beginning with a "v". Vosotros sounds like "bosotros" to me. I kinda say a blend of a b and a v within a word like "entrevista". I watch Univisión TV and there is a Vonage phone ad that concludes: Vueno, Vonito, Varato. All a play on pronunciation. Bueno, Bonito, Barato. .
Saludos a todos,
how to pronounce the letter "v"