In a small bowl, combine tomatoes, basil and vinegar.
Cook fettuccine noodles according to package directions and drain.
Melt Tuscan butter in large skillet over medium heat. Add onions and cook until limp (about 5 minutes). Add chicken slices and cook until lightly brown (about 4 to 5 minutes per side).
Divide cooked noodles between individual serving plates. Add chicken slices, and then pour seasoned butter remaining in the cooking pan over the chick slices. Top with tomato-basil mixture.