From what I understand it comes down to a few issues. One, wolves/wolfdogs were never tested in any amount to verify the rabies vacinations effect.
As mentioned, the laws in many states, counties, cities, towns, etc are against the keeping of wolfdogs, or producing them. Those who allow it can be very very strict and specific about having insurance coverage that allows to cover for accidents, in the amounts of 1 million dollars sometimes. Some states allow you to have wolfdogs with special permits issued by the game commission, these are obtained by applying before you get the animal, then waiting for them to come inspect the "proper containment" and husbandry that will be used, you are then subject to random checks by the commission at any time.
Its a shakey subject, both dogs and wolves are the same species as of the early 90s so truthfully the rabies shot should do thge same for both. Many vets must also have special federal or state permits to treat a wolfdog or a wolf, or they can lose their license.
In the best interests of the animals and yourself, check and understand your local laws long before attempting to get one, also do lots of research, most people who have been impressed by someones pet wolfdog and sought their own afterwards, were lied to by their friend, and were actually looking at a well behaved northern breed or northern breed mix. This is why so many wolfdogs are in rescues, either the law caught up to the owner, or the owner could not handle keeping the wolfdog.