Cut aburage in half, open edge to form a pocket. To remove grease, boil in hot water for 5 minutes with cover, drain. Combine dashi, salt, mirin, soy sauce, sake and sugar and bring to boil. Add aburage and simmer (very low heat) gently 1 1/2 hours or until liquid is almost absorbed. Set aside and cool. Wash rice well and cook with 2 cups water.
Combine vinegar ingredients and heat until dissolved, constantly stirring, cool. Sprinkle cooked vinegar over the rice and mix thoroughly with a cutting motion. Fan to cool. Fill inside of aburage with seasoned rice, not to over stuff.
