Combine garlic, 1/2 teaspoon salt, paprika, cumin, and cayenne in a food processor bowl. Process until mixture forms a paste. Add herbs, and pulse a few times to blend. Add lemon juice, vinegar, and 2 to 3 tablespoons olive oil; blend. Season to taste with salt.
In a large bowl, combine potatoes, peppers, and celery. Season with salt, and toss with herb sauce. Transfer to a large shallow baking dish. Scatter tomatoes among the potato mixture. Drizzle 1 to 2 tablespoons oil over top, and cover with foil.
Bake for 35 minutes. Remove foil. Continue baking until vegetables are tender, 20 to 30 minutes. Serve warm.