Late response but maybe it will help someone.
Yep. Pronunciation-wise b = v and a single r not at the beginning of a word sounds more like d, like the sound the t's make in the word 'butter' (US pronunciation) or the d's in 'ladder'. R is only "rolled" when it's a single r at the start of a word or when a double rr.
Although "A qué hora" could literally translate to "at what hour," hora also means time and this expression, idiomatically, is just "at what time" as given. I think one would be very unlikely to say your sentence in English... Although correct, it just sounds odd and too literal to me.
Perhaps this was not your intent, but I don't think the two phrases are interchangeable. If I were to ask "at what hour," I would expect a time on the hour. If I were to ask instead "at what time," I would accept any time. You are absolutely correct that "at what hour" is still used, however.