1. Place first 9 ingredients in a food processor; process until smooth, scraping sides of bowl occasionally.
2. Split tofu block in half horizontally. Cut each half in half diagonally to form 4 triangles. Make a pocket in each tofu triangle by inserting the tip of a small knife into the center of the longest edge. Slice gently back and forth almost to (but not through) outside edges to form a pocket. Gently stuff each pocket with about 1/4 cup shrimp mixture. Arrange tofu in a bamboo steamer coated with cooking spray; cover with steamer lid.
3. Add water to a large skillet to a depth of 1 inch; bring to a boil over medium-high heat. Place steamer in pan; steam 15 minutes or until filling is set.
4. Combine 4 teaspoons soy sauce and 4 teaspoons rice vinegar in a small bowl. Place 1/2 cup hot cooked short-grain rice on each of 4 plates. Top each serving with 1 stuffed tofu triangle. Drizzle each serving with 2 teaspoons soy sauce mixture; garnish with thinly sliced green onions, if desired.