Combine soy sauce, mirin, sugar, fish sauce, scallions, ginger in a food processor and process to a rough puree. Add mixture to a food bag with the pork and seal. Transfer to sous vide, cook 10 hours.
When done cooking, remove and chill thoroughly (ice water bath, 15 min). Can be stored up to 2 weeks in refrigerator if bag remains sealed.
When ready to serve: Remove pork from bag and discard chunks of fat. Warm and strain congealed marinade through a fine mesh strainer. Reduce and simmer (Kenji says 2 Tbs/15 mins but I just reduce until thick). Mix with mayonnaise.
Preheat broiler to high. Slice pork belly into 1 1/2 wide strips, then slice into 1/2-1/3” slices. Transfer to a foil-lined broiler pan and crisp on one side, about 3 mins.
To serve: Steam buns (if you haven’t already). Spread bun with a little mayo, line with lettuce, top with a couple slices pork and a couple pickles. Devour.