I think the community will auto-correct and weed out the reviewers who are not sincere. For example, if a user buys something based on Amazon reviews (of a Vine member, say), and discovers that the review was insincere, he/she will most likely go to Amazon and give a negative rating to the member/particular review, which will probably feed back into the Vine members reputation, which will in turn be used to determine eligibility for the Vine program.

Agreed that it'll take a while for the system to reach the critical mass, it is, nonetheless, a system that will work very well after it reaches the tipping point.