In a mixing bowl, mix together the vinegar, dry zesty Italian dressing mix, black pepper and red pepper flakes. Drizzle in the olive oil, whisking until it's fully combined and the vinaigrette has emulsified.
Halve, the tomatoes, and thinly slice the onion. Cut the cheese into 1/2" cubes.
Add to the mixing bowl.
Toss gently.
Adjust the salt and pepper just before serving, if needed.
Chill for at least 1 1/2-2 hours before serving.
Sprinkle with additional basil to garnish.