For the reviewers (the "Vine Voices"), it looks like this: on a regular basis, you can choose a product to review (based on your past reviews, purchasing history etc., maybe - they don't disclose how that works), and Amazon sends the product to you. Within one month, you have to post a review.
You can post whatever review you want; to become a Vine Voice, you need a reasonably high reviewer ranking (mostly based on "helpful" votes: [1]). The reviews are labelled as written by a Vine Voice.
As a side note, I know a Vine Voice, and they get emails from companies offering to reimburse them for a product in exchange for a five star reviews every few days.
[0]: https://www.amazon.com/gp/vine/help
[1]: https://www.amazon.com/review/guidelines/top-reviewers.html/
Reviewers generally fall into 3 categories.
The small time reviewers who are excited to get free stuff, and want more free stuff in the future. Aside from their delight of free product, which gives a major positive bias before they even unbox, there is also the fact that if they trash a product, nobody is sending them one again.
Then there is the professional reviewer. They monetize their website and youtube channel heavily, and they live or die by getting product to review. If they trash a product, they face a huge risk of not receiving anything from that vendor again.
The third is the fully independent outfit that has a revenue stream completely detached from the product, and often have to buy it on their own. This is very expensive, and not many consumers pay for reviews. Consumer Reports is an example.
As a result, almost any kind of review you find is likely worthless, and should be treated as a paid ad.
Amzn should pay extra for those. Then ban them:)
Not that it would inherently solve the problem or prevent pay for play, but it would be a great strategy for Amazon to have an independent review company.
There are problems, though: 1. How do we trust that there's some objectivity, particularly as it relates to things that compete with Amazon products?
2. How could reviewers possibly cover any significant amount of the vast number of products Amazon sells?
1. Limited users ability to post reviews to say, 3 per year and no more than 1 in any given month.
2. Limit users ability to post more than one review per vendor, per year.
3. Only allow reviews by verified purchasers.
If bots worked, they wouldn't have had to resort to real humans on Facebook to get reviews.