Preheat oven to 350 degrees F (175 degrees C). Grease a 13x9-inch baking dish.
Mix ground beef, ground pork, and egg in a large bowl; shape into small balls. Arrange meatballs in the prepared baking dish.
Stir barbeque sauce, tomato soup, milk, and vinegar together in a separate bowl.
Bake meatballs in preheated oven until the juices run clear, about 30 minutes. Drain fat from baking dish and pour sauce over the meatballs. Continue baking until the sauce is hot, about 15 minutes more.
