I think there’s a small amount of science behind this, as I understand it, the vinegar acts to slow down your digestion, thus lowering the GI of the meal. So your bg rise would be slower. But it’s of more use to T2s (specifically those on diet control alone). And if you do have slow digestion, having a very vinegary meal (lots of chutney and pickled onions…) it might cause low blood sugar, as your insulin may start to work before your food has been absorbed.
As for the weight issue – the longer you take to digest something, the longer your body has to wait for sustenance, the more if it’s stores it will use up. You still feel full (cos your stomach is busy digesting) so you dont eat more (possibly even you eat less) – so you lose weight! [This last is my guessing at how it works…]