Combine the water, vinegars, onion, carrot, salt, pepper, bay leaves, cloves, juniper berries, and mustard seeds in a large saucepan set over high heat. Cover and bring to a boil, then lower the heat and simmer for 10 minutes. Set aside to cool.
Pat the bottom round dry, then rub with vegetable oil and salt on all sides. Heat a large saute pan over high heat, add the meat and brown on all sides, 2 to 3 minutes per side.
When the marinade is cool to the touch, place the meat in a non-reactive vessel (like a 4-quart stainless-steel pan with a lid) and pour the marinade over it. Place in the refrigerator for 3 days. If the meat is not completely submerged in the liquid, turn it over once per day.
After 3 days of marinating, heat the oven to 325ºF. Add the sugar to the meat and marinade, cover, and cook on the middle rack of the oven until tender, approximately 4 hours.
Remove the meat from the vessel and keep warm. Strain the liquid to remove the solids. Return the liquid to the pan and place over medium-high heat. Whisk in the gingersnaps and cook until the suace has thickened, stirring occasionally. Strain the sauce through a fine-mesh sieve to remove any lumps. Add the raisins, if desired. Slice the meat and serve with the sauce.