I completely agree with you about refusing to vaccinate your pup again...are there any holistic vets in your area at all that you could use?
I also give my dogs coconut oil and I think that in itself is a great preventative for many virual and bacterial infections...plus it's very good for keeping worms and parasites at bay. Not to mention the dental benefits and my boys coats look amazing from it as well. Personally I wouldn't give your pup vit c...although I haven't researched this, but it's my belief that dogs can make their own vit. c. One supplement that I give my dogs is Nupro. Nupro is made with kelp amoungst many other healthy ingredients and it would also be very good for using as a preventative. You should check it out. I also give my dogs extra kelp and a small bit of ACV in their food. My dogs are 4 years old now and I only gave them their puppy vaccines and I don't plan on vaccinating them again. They are very healthy and have white sparkling teeth(from the coconut oil) and gorgeous coats and are very active.