If you could not use rice vinegar (for reasons of food allergy, perhaps), what could you use instead in sushi rice?
I would probably use diluted white cranberry juice, as being the flavour closest to rice wine vinegar. I am not sure if the acidity would be sufficient, however; you may need to add lemon juice. A substitute for soy you will not easily find, as the development of those sorts of flavours requires fermentation.
Sushi is by definition food involving vinegar, so in this case, a substitution would produce a different food. There are several options for writing out the word sushi in Japanese, and instructively, one of the options is 酸し. A form of sushi consistent with its history and origins but not involving the application of vinegar directly would involve fermenting rice and fish together and allowing acids to form naturally (like narezushi or oshizushi). Those acids from fermentation are mostly likely mostly acetic acid anyway, just like you'd produce in sourdough fermentation, so the distinction is almost moot.
But words are somewhat flexible entities, and you could fairly make the case that お酢 refers to acid in the abstract, even from a linguistic perspective. In fact, yuzu juice is, in some reasons, sold under the name ゆず酢, even though the actual product is straight pressed juice, not yuzu vinegar. There are also yuzu vinegars, but at least idiomatically, in some regions, it's within the realm of imagination to consider alternatives, however unlikely. Notably, to the best of my knowledge, Tokyo is not in one of the regions that uses the word "yuzu-su", and Tokyo is the historical center of modern sushi (e.g. the non-fermented type) as we know it.
If you chose the abstract idea of acid, other types would be your substitute for vinegar. Verjuice, yuzu juice, possible sudachi juice, daidai (similar to seville orange) juice, all in dilute forms, might be worth experimenting with, but I'd be disinclined to call the result sushi.
As for soy sauce, in Japan, there is a sesame seed-based soy sauce substitute meant for the narrow market of soy allergy sufferers in Japan, but you'd essentially be unable to eat out anywhere in the country if you were dependent on it.
I cannot find easily rice vinegar in my country. I've found that the rice vinegar in a bit lighter than other vinegars,
Given that I use about 2 part of cider vinegar and 1 part of water. But if the use of any kind of vinegar is a problem I think that diluited acidic stuff (like lemon juice + water) should reproduce the taste (you can do your experiments).
What about lemon juice and wasabi?
(I don't have any experience with the allergy to base this on, but some quick searching indicates these would be ok allergy-wise.)
I am unable to use soy sauce and use of vinegar is limited because of an allergy to wild yeasts. A recent product on the market is Vege Spread, it would require testing regarding amount of product to liquid in the finished product but may prove useful for your planned recipe. Product is produced by Freedom Foods. Sorry if brand names are not permitted.
You could also use Mirin and a dry white wine at a push. These are both slightly acidic and go quite well with rice.
I agree with the poster above. You really don't even need rice vinegar. I love sushi with or without vinegar. Taste about the same to me. Only time sushi rice taste bad is when they add too much vinegar or its not cooked right. I do like a little sugar mixed with the water in the steamer. Don't think I'd ever miss the vinegar even though I still use a little of that also.
I have an intolerance to yeast, so vinegar is out for me. I love sushi, but cannot use vinegar for the rice, because it causes me a lot of joint pain.
I use salt, sugar and a little wasabi mixed with three tablespoons of water...adding a very tasty filling such as crabstick, cucumber, avocado and a spread of creamed cheese. Very healthy and tasty. Salmon and tuna are good, too, with red pepper, chili, cucumber etc.
If the reason is an intolerance to yeast or trace alcohol, "non brewed condiment" can be used as a (considered lower quality) vinegar substitute. It is in the end a synthetic vinegar.
Just skip the rice vinegar or use any vinegar that you have. The only need for vinegar is to apply a few drops onto rice to make acid taste.
What about just plain Worcestershire sauce? It is diluted white vinegar, has suger and salt... might work?