Alola forms? Meaning that we can't use the regular form of that Pokemon in Sun and Moon?
It'll probably work as an add-on type of thing.
Like how DP introduced Basic Rotom, and in Pt we had access to other Rotoms without sacrificing the normal Rotom.
It is probably different formes that are not interchangeable,
meaning Any other Vulpix you import will still be fire type (and could still be possible to encounter along fire routes),
but the Snow-pix is not able to change back to fire type and vice versa.
Its not the first time a Pokemon with a different forme gains different typing or moveset.
Just look at:
Castform (changes in formes changes typing)
Deoxys (retains typing but changes moveset)
Vivillon (essentially different forme encountered on different 3DS' geographical location)
It's as though Vulpix-Snowpix are a combination of these factors.
Also if you look at AZ's Floette (though officially not revealed),
It is but a different forme (data wise it's just a change in byte, meaning it's still a Floette, but with a different forme value),
and it has drastically different stats (it's base stat total is only one point below Florges).
So it's not difficult for GF/Ninty to implement 'Alolan formes', since the concept of different formes having different types and or movesets are already in place.
It's just pretty cool to see them finally do it.