I think sound changes generally follow a fixed pattern as one moves from one geographical location to another. Unless, there is some kind of forced changes thro' literature and cultural influence, sounds are idiosyncratic to that particular region.
The Dravidian urheimat:
At present, Godavari basin is believed to be the place where the proto-Dravidian language speakers had their proto neolithic cultural centre. The present day Dravidian language in that region, Telugu, commonly shows words with 'v'. So, indeed proto-Dravidian sound is 'v'.
However, this region, eventhough became centre of Proto-Dravidian people, is not the place where Proto-Dravidian language was first spoken. That could be mostly in North-West of the subcontinent. That is the movement of Dravidian speakers from West to East. To understand the sound change from West to East we have to observe the population that moved East to West like Kannadigas, Tuluvas and Malayalees*. There you find the words changing to "b". To continue further if you observe West to East (Karnataka to Tamil Nadu) again the sound changes to 'v'.
Now, consider Brahuis who reside in North-West of the sub-continent. And you observe the words again have "b" rather than "v".
eg1. (Courtesy : StarLing database)
Proto-Dravidian : *vā-[r]-
Meaning : to come
Proto-South Dravidian: *vā-[r]-
Proto-Telugu : *rā-
Proto-Kolami-Gadba : *var-/*vā-
Proto-Gondi-Kui : *vā-
Proto-North Dravidian : *bar-
Brahui : ba-nning
Proto-Dravidian : *viḍ[i]-
Meaning : to leave
Proto-South Dravidian: *viḍ-
Proto-Telugu : *viḍ-
Proto-Gondi-Kui : *viṛi-
Proto-North Dravidian : *bic-
Brahui : biṭ-ing
In all probability, the Proto-Dravidian speakers had 'b' sound during their stay in North-West of the subcontinent. And that makes the original sound 'b' and not 'v'.
*It looks like 'v' in Malayalam is a forced change. Culturally isolated Hindu Malayalee communities in Tulu regions and also culturally insulated Malayalee muslims of Malabar had only 'b' sound and not 'v' sound for most of the words. I think this should be the original sound even in Malayala region just like the Western coastal Tulu/Kannada communities. Of course, most of the Malayalees are Proto-Tulu tribes.
However, if Dravidian speakers migrated to North-West of the subcontinent from some other place then I believe there is no way to find the original sound for that letter. But once they set their foot on India it should be 'b'.