Preheat the oven to 400°F (200°C). Cut four sheets of aluminum foil about 17 inches long and set them on a work surface.
In a bowl, toss the sliced zucchini, sliced yellow squash, the thinly sliced shallot and the minced garlic with 1 tablespoon olive oil; season with salt and pepper.
Divide the vegetable mixture among the four foil sheets, placing each pile in the center of a sheet.
Brush each salmon fillet with 1 tablespoon olive oil and season both sides with salt and pepper; place one fillet on top of the vegetables on each foil sheet.
Drizzle 1 1/2 tablespoons lemon juice over the salmon fillets.
In a small bowl, combine the diced tomatoes, the chopped shallot, chopped thyme, dried oregano, dried marjoram and the remaining 1/2 tablespoon olive oil; season lightly with salt and pepper.
Divide the tomato mixture evenly over the salmon fillets.
Fold the sides of the foil inward and then fold the ends to seal each packet, leaving a little air space above the ingredients.
Place the foil packets on a rimmed baking sheet and bake in the preheated oven for 25–30 minutes, until the salmon is cooked through; cooking time will vary with fillet thickness.
Carefully open the foil packets and serve the salmon and vegetables warm.