I learned long ago to not distinguish B and V. For example, beber and vivir look very different in writing yet differ in phonemes only in their vowels.
However, a Colombian friend insists that initial V is sometimes like English V rather than B. This occurred when we were comparing Italian and Spanish and I commented that although written the same, vino was pronounced differently (when in isolation). She claimed that the Italian and Spanish sound the same: that it starts with an English like V even when said alone.
Oddly, although she is quite aware of the English B / V difference, she cannot distinguish English S and Z. To her, the S in mismo is just like S in any other Spanish word.
One possibility is that her knowledge of English has influenced her perception. Another is a phenomenon called phonetic self-deception. I am quite aware of this from English. My variety of English is non-rhotic i.e. we only pronounce R if a vowel follows. For me, sauce and source, are homonyms. Most other speakers are quite unaware of this; they believe that they are pronouncing R since many pairs of words that differ only by a written R do not sound the same e.g had and hard. What is really happening is that the R is changing the preceding vowel.
Might this be the explanation, she is so aware of the written difference that she sees a difference in speech as well? Might her knowledge of English (fluent but not native) have influenced her?
To make this question more clearly about Spanish than linguistics in general: are some native Spanish speakers influenced by the standard orthography into believing that they distinguish B and V even though they don't?