Layer half the plums into the bottom of a very large jar with a lid, pressing the plums down as tightly as possible. Pour half the sugar over the plums, then half the vodka over the sugar. Sprinkle on half the vanilla extract, and place about half the orange peel pieces into the jar. Layer the rest of the plums into the jar, pressing them down, and top them with the remaining sugar, vodka, vanilla extract, and orange peel pieces. Screw on the jar lid.
Using an unscented candle, drip a coating of wax around the jar lid to prevent leaking. Cover the jar with a dark-colored towel. Place the jar on its side in a dark, cool place, and turn the jar every day for 21 days, replacing the towel to keep out light.
After 21 days, strain out the liqueur from the plums and peel with a fine strainer lined with cheesecloth, and bottle the liqueur in attractive bottles with tight-fitting caps.
