So glad to hear your bunny is better. I have 5 buns of my own and and volunteer at a rabbit sanctuary for 5 years as well. I have never heard of giving a bunny Tang. I would seriously doubt that a sugar based product is good for the bun. I have done extensive reading on rabbit diet, and high sugar fruits, grapes among them, are strongly discouraged, as are cereals, corn and bread. Please go to www.rabbit.org and view House Rabbit Society page, it is excellent source of bunny info.
Nor should bun have alfalfa...too much calcium in it which can cause liver damage. Buns should only have unlimited timothy hay, fresh veggies, and, contrary to the pet food manufacturing industry, NO pellets. No cedar or pine shavings in their litter box. The shavings when mixed with the urine produces fumes which have been proven to cause liver damage and death.
You are right in having fresh water available but buns as a rule don't drink a lot of water
anyways. Do not force water on the bun.
I would also recommend you find another vet, one who specializes in exotics such as rabbits. Your vet may mean well but without the speciality training she could do more harm than good. Best wishes.