I write such reviews - both for Amazon Vine and for vendors who offer me free or discounted products. I take my reviewer role seriously and don't treat a review any differently if I paid for the item or not. I recognize that that there is a serious abuse problem - my fellow reviewers use the term "coupon queens", though these can be both male and female - and I applaud Amazon taking this position even though it means I will receive fewer items to review.
I would urge you, though, not to automatically downvote incentivized reviews. If you believe the review is genuinely not helpful, ("I haven't received it yet but I'm sure my grandson will like it, unless I sell it on eBay first..), downvote away. But there are good reviewers out there trying to help purchasers as if they had bought the item themselves. Indeed, those who paid for an item are often biased in favor of it so as to not appear foolish for having spent the money.