Personally, I think it would depend on if it's written vlog or v-log.

I learned the pronunciation from someone who did daily vlogs for like 8 years from the very beginning. Plus, having heared how blogs were pronounced, I assumed it was just a different first letter. They don't call it b-logs, but blogs, hence vlogs and lot v-logs.

But I've heard it pronounced v-log more recently from other sources, and it makes sense I guess.