Place The Roast, Onion, Bay Leaves, Black Pepper, Garlic Powder, And Vinegar In Slow Cooker; Cover Ingredients Completely With Water. Cover And Cook On High Setting Until Meat Is Very Tender And Falling Apart, About 4 Hours. Remove Meat From Slow Cooker And Discard Liquids. Return Meat To Slow Cooker; Shred With Fork And Knife. Stir Tomato Sauce, Chili Powder, And Salt Into The Shredded Meat. Cover The Cooker, And Cook Meat And Sauce On High For An Additional 2 Hours.