Heat gas or charcoal grill. Cut 4 (18x12-inch) sheets of heavy-duty foil; spray with cooking spray.
In medium bowl, mix cheese sauce and cheese. Stir in stir-fry vegetables and potatoes. Arrange 4 sausage pieces and 1 cup potato mixture on center of each foil sheet. Bring up 2 sides of foil so edges meet. Seal edges, making tight 1/2-inch fold; fold again, allowing space for heat circulation and expansion. Fold other sides to seal.
Place packets on grill. Cover grill; cook over medium-low heat 20 to 25 minutes, rotating packets 1/2 turn after 10 minutes, until potatoes are tender. To serve, cut large X across top of each packet; carefully fold back foil to allow steam to escape.