I would definitely change your kitty's diet, but most assuredly not to a Hill's product. How do you feel about feeding raw? There are some good frozen commercial diets you can start with, and if that goes over well, perhaps you could make your own. It can be challenging to convert an older cat over to healthier food, but not impossible. In the meantime, work to eliminate all kibble from the equation, as that is the number one cause of digestive issues in cats. What wet food have you started feeding? I'd suggest for canned foods that you look at something with a novel protein source and no grains, along the lines of Evo 95% Venison or Nature's Variety Instinct Rabbit.
The B12 injections are good, although they're typically used for chronic diarrhea cases more so than vomiting. What is your cat's stool like? What are the other meds that the vet is prescribing?
Here are some links for you to check out regarding diet and how to switch to a better food: