I am sorry your other bunny passed. Was the bun from the same litter, breeder, or pet store? Did the other bunny have the same symptoms? I doubt it is anything you did. Rabbits who are kept in cramped conditions, weaned too early (before 8 weeks), or don’t receive fantastic early care can very easily get sick, and it’s not usually detectable until later on. You did the right thing by seeking vet care, I just don’t know how rabbit savvy that particular vet is…
Have you called around for other exotic vets in your area? It’s very important that the person you go to is experienced with the current needs of house rabbits. Vets who specialize in livestock or breeding rabbits, although not physiologically different from house rabbits, will have very different ideas of what is beneficial. It’s unfortunate that you have to search during a time of illness, but once you have a really good vet you are comfortable with, you won’t need to worry next time vet care is needed.