Combine vinegar, olive oil, salt and pepper in a medium bowl, whisking well to blend.
Next, add tomatoes, basil leaves, and mozzarella cheese. Toss to combine.
Serve immediately (see notes).
Notes
If you wish to prepare this ahead of a party, follow all the directions except leave the basil whole. Tear and toss into salad just as serving. Basil will turn color very shortly after it's torn or chopped for the salad.