Why roast beets in vinegar?
I think it is mostly for the taste. It certainly isn't necessary - I roast beets without any vinegar all the time, and they turn out great. I just roast them in their skins, without any oil or anything added. It takes a long time, over an hour depending on how big they are. When they are completely tender, let 'em cool, and the jacket slips right off. Roasting intensifies the flavor by caramelizing some of the sugars and removing moisture. You can always season with vinegar after roasting, depending on what you plan to do with them.