COMBINE water and bouillon in small bowl; stir to dissolve. Stir in vinegar and sugar.
HEAT oil in large skillet over medium-high heat. Add beef; cook on each side for 3 to 5 minutes or until desired doneness. Remove from skillet. Slice beef into strips. Divide baby greens and tomatoes among plates; top with beef.
ADD sauce mixture to skillet; stir to loosen brown bits from bottom of skillet. Cook over medium-high heat, without stirring, for 5 minutes or until reduced by half. Pour over salad.