Put all the ingredients except for the kombu into a bottle and stir well. The sugar will settle on the bottom, so mix thoroughly.
Next, cut the kombu into 2 cm squares, add to the bottle and let sit overnight. After 24 hours, remove the kombu.
This vinegar should last for up to 1 month in the refrigerator.
Addendum: When making sushi rice, use about 2 tablespoons sushi vinegar to 1 rice cooker cup worth of cooked rice. Adjust tartness to your liking.
Here is chirashi-zushi I made using this sushi vinegar recipe. Refer to Recipe ID: 1659761.
I also recommend my Chicken Chirashi-zushi recipe. Recipe ID: 1788503.
Here is sushi that you wrap in either sangchu (Korean lettuce) or lettuce in my Green Sushi, Recipe ID: 1714958.
And a simple sushi dish I made using canned mackerel, Chirashi-Zushi with Canned Mackerel, Recipe ID: 1350881.
Story Behind this Recipe
This sushi vinegar using lots of kombu is full of umami, and is perfect for chirashi-zushi and hand-rolled sushi.
Since the point is to preserve the sourness of the vinegar, the ingredients should not be heated, and so the sugar must dissolve (not melt). Please be sure to mix well in the beginning, then mix again the next day.