You will no longer be confused if you remember this: “Rice Vinegar + Salt + Sugar = Sushi Vinegar”
It is Sushi vinegar that you want to use to flavour the cooked sushi rice. If you use just rice vinegar, the flavour would be very flat as it lacks the vital seasoning of salt and sugar.
When you are shopping with the intention of buying the “pre-mixed sushi seasoning”, then what you need to look for is “Sushi Vinegar”. As you say, likes of Mizkan sell both “Rice Vinegar” and “Sushi Vinegar” so you just need to pay attention to what it says on the label.
If you can’t find off-the shelf Sushi vinegar, you can always make one yourself!
My Sushi vinegar recipe, which comes from another Sushi chef, is: Rice Vinegar (60ml or 60g) + Sugar (60g) + Salt (10g). This quantity would be enough to season about 300g of raw rice.
So the ratio of Rice Vinegar:Sugar:Salt is 6:6:1. Of course, you can change the ratio to your taste!
You just need to combine them together in a pan and heat it up so that the sugar and salt dissolves. (If you are preparing this well ahead, you don’t need to heat it up: just leave everything in a large jar, leave it in a cupboard and let the sugar and salt dissolve naturally.)
Some chefs add other extra items, such as a piece of Konbu (kelp) and Mirin (sweet Sake) for more depth of flavour. You don’t have to!
The amount of Sushi vinegar you need to flavour the cooked rice is approximately 20mls (of grams) of Sushi vinegar for originally 100g of raw rice. So you need 20% of Sushi vinegar in weight of the raw rice. If you cook 500g of raw rice, then you need roughly 100mls or grams of Sushi vinegar to flavour it. It’s important that you pour the Sushi vinegar and mix it with the cooked rice whilst it is still piping hot. Then when you taste the just mixed rice, you might think “Oh, it’s a bit sharp!”, but do remember that the flavour, or the vinegariness, will mellow down as the
rice temperature goes down. So you want to have the flavour on the stronger side to start with.