Nin hao. In Spain: There are two pronunciations of the letters "b" and "v". a) After a pause (point) or after a "n" or "m", it is an occlusive bilabiate voiced consonant, like in "cambio" or "enviar". The two lips join totally and then are separated. b) If not after a point or "n" or "m", it is a fricative consonant. En "la vida" or in "saber", the two lips do not join totally. So, in "viva la vida", the first "v" is occlusive bilabiate voiced, and the next two are fricative. But in "yo vivo la vida", all the "v" are fricative.

In Spain: There is only a slight difference between the fricative "v" and the occlusive "v" . If you pronunciate all the "v" as if they were occlusive, a spanish speaker would understand you perfectly, although of course he would hear a very slight mistake. I think you would have to be worried about this question only when you had a very advanced level in spanish.

Some speakers pronounce both the V and B as B (many --- if not most --- Latin American speakers). Some pronounce the V like the English V, and the B *similar* to the English B, though not exactly like it. I have heard some speakers pronounce the B like a V ("vien" instead of "bien"). The people I've heard do that are from Mexico, although there might be others that do that, as well. As to your question, I would pronounce all the V's in your sample sentences as a B.

about "las chicas vienen a las tres" the 'v' sounds just like in english. the sound of 'v' has to be soft. i dont know if you have any spanish teacher, but im from argentina and in my opinion 'b' and 'v' sound the same in english as 'b' and 'v' in spanish. :)