Research the differences between Zantac and Prevacid. Be certain you want her on Prevacid first before you push the issue with the pedi. Some meds are blockers and some are actual acid preventers. My son was on Zantac for several months and the dosage had to be adjusted quite frequently as it is based on weight of the baby.
Also, how many times a day do you give her the Zantac and when? It might help to give her a dose of Zantac an hour or so before the last feeding. That seemed to help my son with the vomiting. You have to give the medicine a little time to work before you put something on their stomach.
I know you have been told this before but burp, burp and burp her some more. If you don't get every little bubble out of her tummy, she will throw up.
How much are you feeding in her nighttime bottle? Is it more than her other bottles? If so, it might be just too much on her stomach and that's why she is throwing up that bottle and not the others.
You can also talk to your pedi about changing her formula but honestly that doesn't work very often. Another suggestion to ask the pedi about is adding a little bit of Maalox or Mylanta to her meds. I think they need to be a certain age before you should give it to them, but when I went thru this with my son, a lot of the Moms I talked to swore by giving it to them before their last bottle at night.
Good luck! I know how hard it is to watch them go thru this and not be able to make it go away. Hang in there and definitely make an appt with the pedi to discuss any and all options for her. And if you don't like the answers this pedi is giving or feel they are not doing enough, then by all means go to a different one! You are her mother and her advocate and YOU know what is best for her!