Set the vegetables in the bottom of a medium to large slow cooker, with the potatoes on the top layer. Smash the garlic with the flat of a large knife against a cutting board, and discard the peel. Sprinkle the smashed garlic over the vegetables.
Coat the roast with the rub. Set the roast on top of the vegetables, and pour any excess rub over the roast.
Pour the marinade and soy sauce over the roast. Pour the water around the edges of the pot (not over the roast).
Cover and turn the heat to low. Cook 7 to 8 hours, or until the vegetables and meat are tender. Degrease if needed. Serve in shallow bowls, with a bit of meat, vegetables, and broth (and bread for sopping up the juices).