Isn't it curious how the standard answer is always "cause unknown". And yet diseases and cancers are all too common and acceptable these days. Every other dog owner I talk to has a pet with disease, cancer, allergies, etc. and I can't believe all the medications these are all on. Seems nobody questions it (not you) and accepts the bills and drugs.
For you to question vaccinations is a smart decision. Another big contributor is commercial foods...along with flea medications, heartworm, processed rawhides, pig ears, and all the like. These are all toxic, chemically made, and dangerous. These build up in their system use after use.
On food..there is no such thing as "high quality" food. The foods that say all natural and no preservatives are really not natural. They are a step above grocery store and vet foods, but not biologically appropriate for our carnivoure/meat eating/animal eating dogs. Thier bodies/organs/eyes/intestines/skin/teeth are designed to eat whole raw foods. This includes mostly raw meaty bones, some organ meat, a little veggies, fruit, etc. Same stuff they eat in the wild. Domestication, and the invention of dog food (50 or so yrs ago) has not changed the insides or the nutritional needs of the canine. All of the processed, commercial foods are mostly grain based. They are not designed to eat grains. Grains digest at double the rate meaty bones do. Grains cause allergies, arthritis, and other degenerative diseases. Grain based diets grow the dog at a faster rate (contributes to hip displaysia and more)
Vaccinations are also a cause for disease and immune disorders. Dogs fed a natural diet (raw foods) have a healthier immune system, bones are denser, more muscle mass, etc. etc.
Preparing a raw diet takes only minutes each day. And it's mostly cheaper than a lot of bagged foods. As for supplementing...the only supplement I use is Omega-3 (fish oil) that is necessary for my pit's skin and fur.