Mix the vinegar and water in a small bowl and dip your hands in it to prevent the rice from sticking to them.
Roll a chicken egg-sized amount of rice between the palms of your hands so that it forms an oval. Place on the work surface and make several such rice balls. Place the toppings in front of the rice balls and spread a little wasabi paste on each rice ball.
Place the topping on the rice ovals and press gently.
Arrange on a plate and serve with soy sauce and pickled ginger.