Acetobacter require oxygen to live. That's why we don't get that vinegar formation in the primary, because there is a blanket of CO2 over our beer. Racking it to secondary still isn't a problem if we don't leave much space above the beer (fill carboy clear up to the neck) because the dissolved CO2 will offgas enough to fill this little space but if leave too much space above the beer, there may be enough oxygen for acetobacter to start. Acetobacter eats alcohol so it won't be growing until you have fermentation.
If you added water later in the ferment, that could carry enough oxygen in for something to start but I doubt it could get very far. That may be where you are though. I just don't have enough experience to help you any more. One thing that people mention, if you have an acetobacter infection, it will always get more vinegary while acetaldehyde will gradually get better until it is gone. For now your route to make sure which it is will simply be to let it sit longer. If it gets better, bottle it. If it gets worse, you have malt vinegar to use in cooking. Lots of it.