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.