ACV isn't a fasting issue, but I'll pop in here and give my 2cents worth.
Vinegars are not sugary - the fermentation process makes fermented foods and drinks excellent for those with blood glucose issues (for everyone really). It's to do with maintaining a healthy gut biome and digestive system, which is part of our blood glucose regulation system with various chemical/metabolic actions happening along the digestive journey.
I myself go through a bottle of my favourite ACV fairly regularly. I hate to drink it. So I have it in sauces and salad dressings combined with low-carb mayos and aioli - yum! My mint sauce made from merely mint and ACV is very nice, and as I eat a lot of lamb (I live in Aotearoa/New Zealand) , I go through that reasonably fast.
I encourage my adult children to take ACV regularly, as gallstones run in my family, and me no exception. I kept my gall bladder and the frequent taking of ACV (the acids in the apple as well as the vinegar per se) are the thing that does the trick. One of my brothers had pancreatitis and I read recently that that can come from gallstones - it did not surprise me taking a look at diagrams of our digestive systems, and our pancreas features highly for those of us with diabetes, or a tendency to get it.