To add vinegar to rice to avoid spoilage you can add a small amount of vinegar to the rice either while cooking it or after cooking it.
During cooking you can add a teaspoon or less of some white vinegar for each cup of rice to the rice during cooking.
While you add the vinegar to the rice while cooking you stir the vinegar into the boiling water before adding the rice.
If making sushi rice, you can add a specific mixture of rice vinegar, sugar and salt after the rice is cooked.
For sushi rice, the vinegar mixture is added to the hot rice and stirred into it thoroughly.
The acidity of the vinegar prevents spoilage of the rice by inhibiting bacterial and mold growth which are the main causes of rice spoilage.
When washing rice you are supposed to wash the rice until the water is clear.
Washing or rinsing the rice until the water runs clear is a a common practice when preparing rice for cooking.
When you rinse or wash the rice until the water runs clear it helps remove any excess starch and debris from the rice, which results in the rice being fluffier and also less sticky when cooked.
Some people avoid rinsing the rice or washing the rice until the water is completely clear, but it's a common and recommended way of preparing rice for cooking.
When you do wash the rice until the water is clear it improves the final texture of the cooked rice.
I always wash my rice until the water runs clear and it really does make a difference.
However if you are cooking the rice for a dish that requires a stickier texture such as paella or risotto then you want to rinse the rice less thoroughly.
But for many other dishes or when just cooking rice to eat you should ideally wash the rice until the water is clear but it's okay if you don't do so.