1. Mix the Nuoc Mam sauce, vinegar, onion, juniper berries, fennel seeds, and garlic thoroughly in a large bowl and rub well into the duck portions. Cover the bowl with some plastic wrap and leave in a refrigerator for 8 hours, turning occasionally.
2. Preheat the oven 425 degrees. Drain and reserve the marinade. Place the duck portions, skin side down, in an ovenproof dish. Put in the oven for 30 minutes, basting at least once. Turn the duck portions over, baste and cook for a further 30 minutes, basting at least once. Switch off the oven but leave the duck in it.
3. Spoon 1 cup of the marinade into a hot pan, cover and allow to simmer for at least 5 minutes. Strain, whisk in the yogurt seasoning with salt and black peeper to taste.
4. Server the duck on a dish garnished with the watercress. The sauce is served in a bowl. If guests are not proficient with chopsticks, the duck should be chopped into bite-sized pieces.