Place rice in a sieve and submerge in a large bowl of cold water. Rinse, gently rubbing rice, under cold running water until the water is no longer cloudy. Drain, and set aside until partially dried, about 20 minutes.
Transfer rice to a rice cooker and add water. Cook for 30 minutes, and turn off. Let stand, without removing cover, for 30 minutes.
Meanwhile, in a small saucepan combine 3/4 cup vinegar, salt, mirin, and sugar. Simmer until sugar dissolves. Add konbu, and remove from heat; let cool. When cool, add remaining 1/4 cup vinegar.
Transfer steamed rice to a rice tub. Pour over about 2/3 cup vinegar mixture. With a rice paddle, mix using a slicing motion. Let stand until cool enough to handle.