Drizzle vinegar over hot cooked rice; gently fold to mix. In a small bowl, pour the soy sauce over the avocado toss to coat. Place the cold water in a separate bowl, mix in salt; dip you hands in the bowl of water before shaping each rice ball. This will ensure that the rice sticks to itself, without sticking to you. Place 1/2 cup sushi rice in your hand, make an opening in the middle of the rice, place avocado in the center, cover with rice to form a ball; roll in sesame seeds.