Yes, leave them, this is ideal. Plants take up ammonia/ammonium as their preferred form of nitrogen and they can use quite a lot of it, the faster growing species anyway. Nothing wrong here.
No. First, vinegar is an acid but it will poison your fish. Second, you do not want to be fiddling with pH adjustment, ever, in a tank with live fish. Fluctuating pH, which will inevitably result from this, is far worse and can kill fish.
The pH is tied to the GH and KH. The higher these, the less the pH will move naturally. And the KH serves to buffer pH so it will bounce back to where it normally is in the source water.