In a large bowl, whisk together the pesto, vinegar, ¼teaspoon salt, and a pinch of pepper. Gradually whisk in the oil until well blended.
Cook the orzo
Bring a large pot of water to a boil. Add 2tablespoons salt and the orzo. Cook, stirring occasionally to prevent sticking, until the pasta is al dente, according to the package directions. Drain, rinse under cold running water, and drain again. Add the orzo to the vinaigrette and toss to coat evenly.
Assemble the salad
Add the chicken, tomatoes, and spinach to the orzo, toss gently to combine, and serve.