Some of the least useful reviews I've read on Amazon have been Vine reviews. Not all of them, but you can tell when someone only got the product because they could get it free and only reviewed it so they could get more free stuff in the future. Despite their ignorance of the category and lack of a need or desire for the product (someone who only shaves with blades reviewing an electric razor, for example), these reviews are often top ranked just because they showed up early and weren't illiterate, crowding down better reviews below that provide more useful information towards people actually trying to decide between that razor and others like it.
Your reviews are only going to be as good as your community, and Amazon's community is mixed at best. There's probably nothing they can do to fix that. Any rules you make will be sidestepped by those determined to sidestep it, and will block honest and good reviewers who won't bother trying to clear your hurdles. The quality of their reviews will not improve as a result of this effort, however well-intended it is, and I wouldn't be surprised if there were a slight net decrease in quality.
I actually don't buy a lot of PC stuff these days, but when I did, Newegg's reviews were pitch perfect. Even when I wasn't buying from newegg, I always checked their reviews. Their community didn't just write useful reviews, they were very good about getting the most useful reviews to the top of the list. You'd be very hard-pressed to try to replicate their community building success, and if you did, it'd be the same way they did -- blind frakking luck. Amazon's attempts to regulate the area ignore the truth, it's not about the rules, it's about the people. Regs won't deter the bad actors but they probably will discourage the good ones.