COMBINE broccoli, cauliflower and carrots in large microwaveable casserole. Add water; cover with lid. Microwave on HIGH 5 to 6 min. or until vegetables are crisp-tender.
MEANWHILE, cook bacon in small saucepan on medium heat until crisp. Remove bacon from pan; drain on paper towels. Discard drippings from pan. Add VELVEETA and onions to pan; cook on low heat 1 min. or until VELVEETA is melted and mixture is well blended, stirring constantly. Stir in sour cream and bacon.
DRAIN microwaved vegetables; return to casserole. Add VELVEETA sauce; toss to coat.