Preheat oven to 300 degrees F. Coat an 8-inch-square glass baking dish with cooking spray.
Toss tomatoes, garlic, oil, Italian seasoning, salt and pepper in a medium bowl. Spread the tomatoes in the prepared baking dish. Bake until the tomatoes are broken down and the juices are thick and syrupy, 1 hour 20 minutes to 1 hour 35 minutes.
Transfer the tomatoes to a blender. Add vinegar and puree. (Use caution when blending hot mixtures.) Cool completely before using.
Make Ahead Tip: Cover and refrigerate for up to 1 week. Stir before using.