Peel the roasted beets. Slice lengthwise, then crosswise into half-moons. Make a marinade of vinegar, sugar, olive oil, salt and pepper. Toss with beets and marinate in refrigerator 2 to 3 hours. Drain yogurt in a cheesecloth -lined strainer for the same 2 to 3 hours.
Combine garlic and yogurt. Stir in half the dill and a little of the beet marinade if desired. Drain the beets. Toss with the yogurt mixture. Sprinkle with remaining dill.
The beets can be roasted and marinated 4 or 5 days ahead.