I first noticed them this morning - a row of five stars below some articles, just underneath the Facebook stuff. Scroll your mouse over them and you find that they're ranked from "Bad" through "Boring", "Ok", "Good" and Excellent. Clicking the "?" link shows that the ratings are run by a company called Outbrain, and it appears that you need an account with them for some reason.
I won't be signing up for anything like that, I'm afraid, but it does intrigue me. Is this a new dimension to the Vine? Have you seen it? So far it hasn't been on all articles or seeds, and I've not seen it often enough to work out if there is a pattern to the ones that do sport it.
UPDATE: See the official explanation in comment #11 below.