Never heard of using vinegar for anything but testing rocks and cleaning tanks. And even in those two applications you are suppose to rinse throughly. With that in mind I wouldn't do it.
some where around 3-4 pH level. remember vinegar is for cooking only please. and sometimes it is rememdy to clean tanks. since it is acidic it has a low pH. High pH means the water is highly alkaline.
One of the wholesalers I sell my fish to uses Hydrochrolic acid to adjust their well water which comes out highly alkaline to a slightly acidic PH of 6.8 for all their tropical tanks. Still others use citric acid for the same purpose.
HCL is even more acidic than white vinegar. The key is knowing how much to use to reduce the PH to your target level.
Having said that, it is still good to reconsider why you need to reduce or change your PH. Is it really necessary for your fish?
Some amazon fish thrives in PH of 4.0. But they can be acclimated to higher PH when done slowly and live in the higher PH for a very long time.
Vinegar can in fact be used to lower PH .. how ever i would not do this, we in the pond bizz use vinegar to help cure the lime factor in concreate made ponds but not to lower PH, by cureing the lime in concreate it make the pond so the PH will stay lower,, use somthing that is made to lower ur carbonate hardness the ur PH should maitain to where you set it
never try vinegar , try peat filter medias ,naturally softens, and purifies your water ,highly effective, natural method of softening and stabilizing shld bring ph down to 6.0-7.5.