I found out from my vet that any kind of pork product can induce seizures, even in dogs that aren't epileptic. I stopped giving my dog ham bones (he used to get a lot) and pork and he has not had near as many seizures. Also, I only give him spring water because tap water has chlorine, which is another seizure trigger.
Another trigger that some people may not think of is exposure to fertilizers, herbicides, and pesticides. Even if you do not use these products in your own yard, your neighbors could be spraying them.
As far as the vaccination issue goes, my dog also had his first seizure about one week after his rabies booster.