In a large bowl, whisk vinaigrette, barbecue sauce and mustard until blended. Reserve 1/4 cup mixture for basting. Add beef to remaining mixture; toss to coat., Alternately thread beef and tomatoes on 4 metal or soaked wooden skewers. Lightly grease grill rack., Grill skewers, covered, over medium heat or broil 4 in. from heat 6-9 minutes or until beef reaches desired doneness, turning occasionally and basting frequently with reserved vinaigrette mixture during the last 3 minutes.