Whisk together soy sauce, mirin, vinegar, sesame oil, sriracha, if desired, and cornstarch mixture in a bowl.
Warm a large skillet over medium-high heat. Add grapeseed oil. When it shimmers, add mushrooms. Cook, stirring occasionally, until mushrooms release their liquid and just begin to brown, about 8 minutes. Add bell pepper, carrots, snow peas, and scallions and cook, stirring often, until just tender, about 1 minute. Stir in bok choy; cook, stirring, for 30 seconds.
Stir in ginger and garlic and cook, stirring, until fragrant, about 30 seconds. Push vegetables to one side of skillet, add shrimp to skillet, and stir-fry until just opaque, 1 to 2 minutes. Add 2 to 3 Tbsp. water to skillet and quickly stir to loosen any browned bits from bottom of skillet. Stir-fry, mixing all ingredients, just until water evaporates.
Reduce heat to medium. Quickly whisk sauce and pour into skillet. Cook, stirring, just until sauce begins to thicken and coat shrimp-vegetable mixture, 1 to 2 minutes. Divide rice among 4 shallow bowls, if desired. Spoon sir-fry mixture on top. Sprinkle with reserved dark scallion greens and sesame seeds, if desired.