Ideally, when roasting beets, the skins should just come right off once they're done and cool. I don't seem to be good at this, though: sometimes they're great, and sometimes the skins are pretty ...
I have always roasted my beets in a bit of vinegar in the bottom of the pan like 1/4" or so. It is how I was taught. But why do it? What does the vinegar do for the beets?