Put water in the sous vide vessel and heat it to 176 degrees Fahrenheit (80 celsius). If your pork belly is too large to fit in your sous vide vessel, cut it in half.
Put the remaining ingredients in a medium bowl and whisk until smooth. Put half of the marinade into a ramekin and refrigerate it. Brush the other half onto both sides of the pork belly.
Put the pork belly into vacuum sealer bag(s) and seal it using the moist food/gentle settings. Place the bags into the water bath for 6-8 hours.
When you're almost ready to remove the pork, set up a gas or charcoal grill for indirect cooking. For the gas grill, turn on some of the burners to medium-high, but leave one set of burners off. For a charcoal grill, put the charcoal on one side of the grill only. If you can, place an aluminum drip pan underneath the unlit side of your grill to reduce the likelihood of a flare-up.
Carefully remove the bags from the water bath. Cut them open to drain the fluid from the bags, being especially careful if you used the red food coloring in the marinade. Pat the pork dry and coat it with the remaining marinade. Place it fatty side up on the unheated side of your grill and cook for about ten minutes, turning once, until it begins to get char spots. To serve, slice it thinly.
See the notes above for instructions on how to freeze half of it for cooking later if you won't need all the servings at once.
Macros per serving (a little over 3 ounces): 479 calories, 1 gram net carbs, 9 grams protein, 50 grams fat. The actual totals on this are likely a little lower, as some of the marinade is lost from the liquid drained from the sous vide bags, but it's impossible to calculate that, so I just included all of it.