Preheat oven to 475°F. Wrap beets in foil; place on a rimmed baking sheet and roast until tender, 1–1 1/4 hours. Unwrap; let cool slightly. Remove skin, then cut beets into wedges. Place in a small bowl, add umeboshi and vinegar, and toss to coat.
DO AHEAD: Beets can be made 5 days ahead. Cover and chill.
