Cut beef diagonally across the grain into thin strips.
Combine vinegar and chili powder in a shallow dish or large heavy-duty zip-top plastic bag; add beef. Cover or seal, and chill 1 hour, turning occasionally.
Remove beef from marinade, discarding marinade.
Heat oil in a large nonstick skillet or wok at medium-high heat 2 minutes. Add beef and fajita seasoning; stir-fry 2 minutes or until beef is lightly browned. Remove from skillet; drain on paper towels.
Add bell pepper strips and onion; stir-fry 4 minutes. Return beef to skillet; stir-fry 2 minutes. Add tomato, and cook until thoroughly heated. Serve with tortillas and salsa.