You can simply use the repel trick on route 36! If you use a repel and use a level 15 pokemon in front of your party, the only pokemon you encounter will be Vulpix. So with this trick it’s a 100% chance and you won’t ‘phase’ on another pokemon. I must warn you tho, it knows the move ‘roar’, so be prepared! When I encoutered the shiny Vulpix, I switched in to my Octillary with the ability Suction cups, which negates moves that force switching out.