Alola Vulpix can be a reward from the Lawyer in Anthinans Park District when you beat Santa. Choose wisley because you can only do it once. (CHRISTMAS EVENT ONLY AND HAS ENDED)
Alola Vulpix can also be hatched from a white egg. This egg can be obtained from the day care by breeding or can also be traded with another person.
Also If you get a deep sea ticket you can trade it with the pirate in the ticket seller at port deca, buy teh fridgia and if you go in the cave theres a chance to find either the alolan vulpix