Um, maybe John Mace is familiar with a non-Andalusian dialect of Spain? I was taught New World Spanish, which comes from Andalusian Spanish. The letters b & v are, I was told, equivalent in those dialects.
I’ve taken classes in Spanish phonetics and linguistics. I was taught this by my madrileño Spanish professor, who did mention that there are certain hypercorrectionalist speakers that create a distinction between <b> and <v>, but this is quite rare and a wholly artificial affectation. I’ve never run into such a thing. It’s certainly not the norm in any Spanish dialect. I’m far more familiar with peninsular Spanish than American Spanish, and like I said, I’ve never heard it done.
If I bump into my Spanish teacher, I’ll take it up with her… 
You sound pertty sure of yourself on this, so I’ll stand corrected. Are you a native speaker, btw? If not, I’d be interested in hearing from a few speakers from different parts of the Spanish speaking world. When I hear someone say “vaca”, I don’t hear “baca”, with a hard “b” like “banco”, but maybe I’m prejudiced by knowing that it’s spelled with a “v”.
When you hear someone say “vaca”, you hear /'ßa ka/. Easy to perceive that as a [v] if you’re not a native speaker. I’m not either, but I have a pretty solid accent, and as I’ve said, I’ve taken a few classes in Spanish linguistics.