Add mushrooms to a bowl and cover with 1 cup warm water. Let sit for 30 minutes to rehydrate. Strain mushrooms, reserving soaking liquid in a small bowl. Rinse mushrooms and set aside. Strain soaking liquid through a paper towel into a medium saucepot and set aside briefly.
For the rice, rinse rice in strainer under cold water. Add rice, 2 3/4 cups cold water and salt to the saucepot containing the mushroom soaking liquid. Cover and let soak for 30 minutes. Remove lid, then turn heat to medium and bring to a boil. Reduce heat to low, cover and cook for 15 minutes without removing the lid. Transfer to a mixing bowl, stir in vinegar, and cover with a clean damp kitchen towel and keep at room temperature.
Heat grapeseed oil over medium-high heat in a saute pan. When the oil begins to shimmer, brown all sides of the sausage. Pour most of fat from pan and return to stovetop. Reduce heat to medium, stir in coconut milk, soy sauce, and oyster sauce. Cover and let cook for 15 to 20 minutes until the sausage is cooked through.
Remove sausage and let rest. Then slice on the bias. Return sausage to pan, add cooked rice, and stir to combine. Season, to taste, with salt and pepper if needed. Garnish with scallions.