Look for frozen Chinese steamed buns at Asian specialty grocery stores. Or, if you'd rather, you can substitute mini pita breads for the buns.
Combine first 3 ingredients in a small saucepan over high heat, stirring until sugar dissolves. Pour vinegar mixture over cucumber. Let stand 20 minutes; drain.
Prepare buns according to package directions.
Combine turkey and hoisin in a bowl; toss to coat. Spread 1 teaspoon cranberry sauce on each bun. Top evenly with turkey mixture, cucumbers, and onions.