Line strainer with paper toweling or cheesecloth. Add yogurt; refrigerate for up to 1 hour to drain the liquid.
Meanwhile, peel and grate cucumber, using large holes on grater, into medium-size bowl. Peel and grate garlic, using small holes on grater, into bowl.
Stir in drained yogurt, dill, vinegar, salt and pepper. Serve with Lamb Keftedes. Makes about 10 servings (2-1/2 cups).