Bring a large pot of salted water to a strong boil over high heat. Add the orzo and cook just shy of al dente, about 1-2 minutes short of the package directions.
Drain the pasta and do not rinse. Immediately transfer the hot pasta to a large bowl.
Add the pesto, lemon juice, vinegar, and salt, stirring to combine. Allow to sit for 5-10 minutes for the flavors to absorb and the pasta to cool.
Add the tomatoes, basil, pine nuts, and mozzarella, stirring to combine. Serve warm, at room temperature, or cold.
Notes
For gluten-free, use gf orzo or any similar small shaped pasta.