It depends on your Vulpix's current level. As Ninetales can't learn any moves upon leveling up, besides Dazzling Gleam, witch is learned upon evolving. Vulpix learns Ice Beam at lv 36, and learns Blizzard at lv 42. Considering both are TM's in the game, you could evolve Vulpix when it is necessary for you. The TM for Ice Beam can be found on Mount Lanakila on Ula'ula Island, before you hit the Pokemon league and the TM for Blizzard can be found at the PokeMart at Seafolk Village on Poni Island, the last island in the game.
So I would evolve Vulpix after it has learned Ice Beam, as it is a game changing move to have Vulpix learn