In a small saucepan over high heat, whisk together the rice vinegar, sugar, and salt until the sugar dissolves. Fold this into your sushi rice while the rice is still warm.
Add the imitation crab to a food processor and blend until finely minced. Transfer to a medium bowl along with the rest of the filling ingredients. Mix well.
Place a sheet of seaweed (shiny side down) on a bamboo mat. Spread 1 cup of rice evenly on the seaweed, except 1 inch at the top.
Scoop some crab filling and shape it into a 1-inch wide log across the middle of the rice. Add avocado and cucumbers.
Start rolling from the bottom side, maintaining a firm pressure on the mat. Right before you reach the end, lightly wet the edge of the seaweed with some water. Roll over the edge and place the roll seam-side down.