If your dog has recently been vaccinated, it is likely to demonstrate aversion to food. It may happen occasionally. However, it is supposed to last for a couple of days. So, it is not easy to decide why the dog is not much interested in eating. Observe for a few days more; it is supposed to come back to its usual appetite. However, such possiblity should not refrain you from inoculating your dog; because vaccination enables dogs fight infections.
It is to be noted here, loss of hunger due to vaccination might be accompanied by some other side effects such as tropidity or slight fever.