“B” and “V” are pronounced the same way in Spanish. What happens in your example with a “v” in the middle of the word (“vívido”) is that, when pronouncing this word in an isolated way, the first “v” and the second “v” might have a slight difference in Spanish, being the first one more “explosive” (completely stopping the flow of air out of the mouth when pronouncing it) and the second one “softer” (upper and lower lips will get very close but air will still come out when pronouncing it). This also happens in other words with “b” and “v” in first and other positions in the same word: vívido, vivir, bebé, bávaro... When pronounced isolated, there will be a slight difference. When pronounced in the middle of a sentence, when both “v” or “b” are between vowels, this difference disappears: “Lo recuerda de forma vívida” (both soft, due to last letter “a” from “experiencia” coming right before) “Ese bebé tiene hambre” (both soft, due to last letter “e” from “ese”). This also happen when pronouncing “d”, in words with couples of “d” at the beginning and in the middle, like “dedo” or “dado”. In this cases, isolated, first “d” will be more explosive (similar to English) and second “d” will be softer, similar to English “th” in “though”. These differences are very very slight, and we Spanish speakers are not aware of them unless we make the effort to notice them. Sorry for the long explanation. Hope it helps.

No existe una diferencia de pronunciación en la norma, porque ya en el latín coloquial que se hablaba en Hispania se unificó hace mucho la pronunciación de ambos grafemas. Hay una anécdota graciosa derivada de esa "confusión" y está en una frase de autor anónimo que dice: "Beati hispani quibus vivere est bibere" (feliz Hispania donde vivir es beber) y se refiere al hecho de que en Hispania las palabras “vivere” (vivir) y “bibere” (beber) sonaban exactamente iguales.

En el habla coloquial se suele pronunciar la V como B porque pronunciar la B es más fácil, requiere menos esfuerzo o énfasis al pronunciar. Pero lo contrario no suele ocurrir casi (solo en algunos países, como Chile suelen "sobre-corregir" y dicen, por ejemplo, "varco" en vez de barco). In formal educated speaking, V and B are pronounced as V and B, respectively. Maybe "móvil", "automóvil" and other words that one would think should be written with a B, are written with a V because in greek the letter beta ("vita", β) is pronounced as a V and for some reason the pronunciation was preserved as such.

I've been told by some native Spanish speakers that confusing b and v in writing is quite common. It's not unusual to see móvil spelled as móbil, for example (I'm not sure why it's spelt móvil actually. Maybe someone can explain it. It seems strange to me to use v rather than b in this case).

En realidad al pronunciarlas no hay diferencia alguna, al menos en la práctica. Uno encuentra la diferencia es en la forma escrita, nada más.