Saccharomyces yeast turns sugars into ethanol. Acetobacter bacteria turns ethanol into acetic acid.

It's all about controlling the cultures you use for the fermentation.

That's why home-brewers have to be so careful about sterilizing their fermentation vessels and boiling their liquids. You absolutely, positively do not want the wrong microbe getting into your booze. If it's a wild one, it could end up synthesizing any number of organic chemicals that can negatively affect the final product.

That's not how it works. Acetic acid fermentation needs oxygen, alcohol fermentation doesn't. You need to prevent fresh air from getting into the vessel, but that's it. You don't boil anything.

Kefir culture, which contains both types of microbe, is perfectly happy turning sugar into alcohol into vinegar inside a sealed vessel. If you wanted to stop it, you would need to evacuate the vessel or fill it with an inert gas. If you also had a microbe in there that split water into H2 and O2, you'd have no chance at all at more than about 0.5% alcohol by volume in there. I'm not aware of any existing kefir culture that has such a microbe in it, but once one was adopted it would be near impossible to get rid of it while saving the rest of the culture.

The boiling happens with beer, mostly. With wine, you're usually relying on the yeast that naturally occurs on the surface of the grapes, so if you boiled (or pasteurized) it, there would be no fermentation unless you added additional yeast. That sometimes happens, because some people prefer consistency to authenticity.

Home brewers use fermentation vessels that allow gases to escape through a liquid trap and the whole process is done at positive pressure. You can bet your ass that those vessels are thoroughly sterilized before each use.

We may be talking past each other here.

Yes, we are. Kefir is made acidic by the lactic acid, not acetic acid. Lactic acid is indeed produced in wine.

Fermentation produces so much CO2 that it quickly fills up the vessel and pushes any remaining oxygen out. People didn't know where yeast comes from until recently. You're making it far more complicated than it is.

Kefir most definitely has vinegar in it, especially if you're getting towards the bottom of the batch, after all the sugars are gone. That's what takes it from "just a bit tart" to "whoa, that's sour".

Though it depends a lot on your culture. The "kefir" sold commercially in the US does not have yeast in the culture, and therefore no alcohol or vinegar. It doesn't really qualify as real kefir. You have to acquire cultures and make your own to get the full experience. You also have to acquire a taste for it.