Sounds like way to much White Vinegar. It should be more like 1-2 Tablespoons per gallon of water.

Keep in mind that vinegar is also used as a herbicide. It will kill plants if it gets on their foliage. Always remember not to get any mixture containing vinegar on the foliage. If you do make sure you wash it off right away.

It does need to be in a sunny location to bloom. As for the yellow leaves, I'm not sure.

Hibiscus leaves i notice cycle. If they go bright yellow, i wouldn't worry too much.
As far as fertilizing goes, do not use a "superbloom" or "rose food" on them. This can lead to symptoms of chlorosis. Instead use a palm food, an example of palm food nutrient ratio is "18-6-18".
It does not have to be that exact ratio, but the first and last numbers should be higher than the middle, too high of phosphorus can lead to an eventual decline in the health of the plant.
Also, make sure the fertilizer has micro nutrients and to keep it on the slightly acidic side. If you drink coffee, throw some of the grounds on the soil to keep the pH in check. Remember our water is hard and on the alkaline side.

Good Luck!

Yes I agree with the palm food. I have had good luck with using this on my hibiscus!