Let butter and cream cheese sit out at room temperature.
Place the piece of salmon on a plate. Season on all sides with a light layer of salt. Let the salmon sit to absorb the salt while you make the poaching liquid.
In a small pot, add the onion, garlic, wine, water, 2 tablespoons of rice wine vinegar, and 2 tablespoons of salt. Bring this liquid to a boil and shut off immediately. Allow the liquid to sit at room temperature for 5 minutes.
Bring the water and wine stock back up to a boil and turn off the heat. Add in the piece of seasoned salmon. Poach the salmon with the heat off for about 5–7 minutes depending on the thickness of your salmon. Remove the salmon from the liquid and place on a plate lined with a paper towel. The salmon should be nicely cooked through and flake easily. Allow the salmon to cool.
In a medium mixing bowl, add the softened cream cheese and butter. Whisk together until thoroughly combined. Add in a teaspoon of rice wine vinegar, the fish sauce, grated ginger, a pinch of cayenne, a pinch of salt, and the dill and scallion. Mix well.
Flake the cooled salmon into a medium bowl with the seasoned cream cheese. Gently fold in the flaked salmon until thoroughly combined. Taste and adjust seasoning if necessary.
Serve immediately or chill the rillettes in the refrigerator until ready to eat. You should pull the rillettes out for 15 minutes to temper before serving.