Thaw the oxtail if frozen. Cover with water, add apple cider vinegar, and let sit for 30 minutes to an hour. Rinse well. Season with salt, pepper, garlic, ginger, herbs, and cayenne. Set aside.
Blend 2/3 of the bell peppers, 1/2 the onion, ginger, garlic, 1 Tbsp. of soy sauce/coconut aminos, and the cilantro. Add enough water to blend it if it's too thick to blend at first. Chop the remaining bell peppers and onion. Mix everything with the seasoned oxtail, and marinate in the fridge 4-24 hours.
Cooking the oxtail
Remove the oxtail from the marinade and set aside. Pour the marinade into a pot and heat on medium (4). Allow to simmer loosely covered.
In the meantime, heat a skillet to medium (5). Once hot, add 1 Tbsp. oil and 2 Tbsp. brown sugar. Let brown sugar melt and stir evenly around pan. Once sugar is simmering, add oxtail. Brown all pieces on all sides, then remove. If necessary, do it in multiple batches.
Once all oxtail pieces are browned, add to marinade. Stir to combine, moving the vegetables to the top so they don't burn. Let simmer loosely covered over low (2-3) heat, stirring every ~30 minutes, for 3-4 hours.
Once oxtail is fork tender, skim off surface oil. Stir in the tomato paste and remaining 1 Tbsp. soy sauce, and adjust seasonings to taste. Let simmer another 20-30 minutes.