If you do that, I recommend you get the Vulpix to Level 100, then evolve it. Otherwise, you have to work on leveling Ninetales for the next 75-or-so levels, and at the rate of 5 per post, you'll be at it for a while!
When a Pokémon evolves, the "posts per level" stays the way it was on the previous species. After it levels up once, it will update to the rate of the new species.
Do you want to just try and catch a new Alolan Vulpix fresh, and work that to Level 100? It will take you less time to get there than if you stuck with the Alolan Ninetales you have now; it takes 297 posts to get something basic from 1 to 100, whereas a Level 26 Alolan Ninetales would take 370. ...Then again, you appear to already be doing so, in which case my advice is happily unnecessary.
I could never see myself doing level 100 for the time being, but maybe if I find a Pokémon I like that might change.
Out of interest, what sort of Pokémon would you like? There are a lot out right now, but is there one not yet released you think you may have your eye on?