Using a food processor, puree the basil, scallions (white and light green parts), yogurt, lemon juice and 1 1/4 tsp. salt. With the machine on, drizzle in the EVOO.
In a bowl, mix the corn, sugar snap peas, lima beans and bell peppers. Add the dressing and toss to combine; season with salt. Top with the dark scallion greens.