Dry fry the soybeans (i.e., do not use oil) on low heat until they are light brown. Soak the kombu in water for around 30 minutes and slice it thinly with a kitchen knife.
Soak the kombu and soybeans in water overnight. In a separate bowl, soak the dried Shiitake mushrooms in water overnight. (Remove the Shiitake stems during the soaking process so the rehydrated Shiitake will become plump.)
Add water to the kombu/soybean Dashi until there is 300ml of liquid, and add 300ml of Shiitake dashi.
Add the kanpyo to (3) and simmer it together with the kombu and soybeans for 2 minutes, removing the scum. Remove mixture from heat and let cool.
Strain the liquid from (4) (300 ml) into a pot together with the Shiitake with their stems removed, and add mirin, soy sauce, and vinegar. Bring to a boil and remove the scum.
Simmer on low heat for 10 minutes. Remove from heat and let cool. Taste the soup and either return it to the heat to evaporate more liquid and concentrate the flavors or add soy sauce to taste.
