Once rice is cooked, let it cool until it is just warm.
Stir in the rice vinegar.
Place a piece to parchment paper on the countertop.
Scoop out a generous cup of rice and place it on the piece of parchment, smoothing it out into a square shape about 8 x 8 inches. If the rice is sticking to your spoon, dip your spoon into some water or use your slightly wet hands to spread.
Take 2 of the pieces of tofu skin and place it on a cutting board. Cut the sweet potato into long pieces like the shape of a bread stick. Place the pieces of sweet potato across the middle of the tofu skin matching the width of the rice layer. Roll the tofu skin around the sweet potato, creating a tube. Pour a little bit of water onto a large plate and add one teaspoon of soy sauce. Roll the tofu skin tube in the liquid until it softens. Place the tube onto the center of the rice. (If using tofu, place the tubular sweet potato pieces onto the rice layer and cut tofu pieces to match, adding them next to the sweet potato)
Sprinkle the toasted sesame seeds next to the sweet potato tube.
Sprinkle the toasted nuts onto the other side of the sweet potato.
Spread a tablespoon of the pickled vegetables over the toasted sesame.
Lifting the two lengthwise edges of the parchment, bring the two sides of the rice together, pressing the edges together to encase the ingredients, creating a burrito shape. Using the parchment to help roll the "burrito", squeeze it gently to make sure the two rice edges seal well and wrap the remaining parchment around the burrito.
Serve while still warm. To eat, either peel back one end of the parchment an eat from that end, or cut it in half at the center and peal back the parchment as you eat.