Heat vegetable oil in a small skillet. Add zucchini or summer squash and cook on medium heat for 5 minutes, stirring occasionally. Add chicken and toss for 2 to 3 minutes, until warm throughout.

Meanwhile, slice rolls lengthwise. Toast open-faced under the broiler for 1 minute, or in a toaster oven.

Toss warm squash and chicken in vinaigrette and place on toasted rolls. Layer with arugula or watercress leaves and tomato slices. Close sandwiches and serve.