Place Beef In The Slow Cooker Crock. Pour Beef Stock, Vinegar, And Worcestershire Over Beef. Cook On Low Until Beef Is Cooked To Medium, 3-4 Hours. Remove Cooked Beef From Broth And Set Aside. Reserve Broth. Preheat An Oven To 350 Degrees F (175 Degrees C). Heat Oil In A Large Skillet Over Medium-High Heat. Cook Mushrooms In Oil Until They Begin To Soften; Add Onion And Green Pepper. Cook And Stir Until Vegetables Are Soft, About 5 Minutes. Slice Beef Thinly. Divide Beef And Vegetables Evenly Among The Rolls. Top Each Sandwich With 2 Slices Cheese; Wrap In Foil. Place Sandwiches In Preheated Oven And Heat Until Cheese Is Melted; About 10 Minutes. Serve Hot Sandwiches With Reserved Beef Broth, If Desired.