Preheat smoker to 250°F. Add 50/50 mixture of apple juice and water to water tray.
In a medium bowl, combine red wine vinegar, soy sauce, honey, garlic salt and pepper. Place duck in a gallon-size resealable plastic bag.
Reserve ½ cup marinade for later use. Pour remaining marinade over duck. Refrigerate and marinate for 2 to 4 hours.
Place duck on middle rack of smoker and smoke, uncovered, for 1 hour, basting several times with reserved marinade.
When well browned, cover with aluminum foil and smoke for 2½ to 4 hours or until internal temperature reaches 165°F.