Put the miso, rice vinegar, garlic, ginger, and 1 cup water in a food processor or a large blender. Cover and process or blend until smooth, scraping down the inside of the canister at least once.
Stir the miso mixture into 4 additional cups water in a medium saucepan. Add the tofu, mushrooms, pea shoots, and scallions; bring to a simmer over medium heat, stirring often. Reduce the heat to low and simmer, uncovered, for 5 minutes. Turn off heat and stir in the sesame oil and soy sauce before serving.
