A reviewer's ranking is determined by the number of helpful votes from other customers.
At least for me, Amazon has been slipping. Product prices have gone up to close-to-retail prices. Quick shipping is great but the cost is baked in to the price more so than the annual Prime membership fee which has been used to extend other prime services that I don't care so much about. And now, I can't trust reviews.
The reviews are now a real problem, though. I think it goes way beyond the Vine Voices program, too - I've seen products with page after page of 5 star reviews, each with a note at the bottom along the lines of "I received this product for free in exchange for my honest opinion". I don't recall these being identified as Vine Voices, they were just "normal" users.
Whether sellers are directly contacting normal users outside of the Vine Voices program, or whether the reviews are just straight up spam or paid shills I don't know, though I'd bet it's a bit of both.
Sellers attempting to manipulate reviews is present in regular reviews as well. It's got nothing to do with the difference between voice reviews and regular reviews. This is a problem Amazon hopefully (and probably) is addressing already with software to detect it.