We wanted perfectly cooked vegetables in a liquid with a saucy, not thin and soupy, consistency.
Using highest sauté function, heat oil in Instant Pot until shimmering. Add bell peppers, onion, and salt and cook until vegetables are softened, about 5 minutes. Stir in garlic, herbes de Provence, and pepper flakes and cook until fragrant, about 30 seconds. Stir in tomatoes and reserved juice, eggplant, and zucchini. Lock lid in place and close pressure release valve. Select high pressure cook function and cook for 1 minute.
Turn off Instant Pot and quick-release pressure. Carefully remove lid, allowing steam to escape away from you. Using highest sauté function, continue to cook vegetable mixture until zucchini is tender and sauce has thickened slightly, 3 to 5 minutes. Stir in vinegar and season with salt and pepper to taste. Dollop individual portions with pesto and serve, passing extra pesto separately.