You must chain breed to get Freeze Dry on a Vulpix. Two Pokémon can pass Freeze Dry down, Swinub and Delibird. The thing about that is Swinub gets Freeze Dry from Delibird as well, so it’s easier to just use Delibird. To get a Delibird with Freeze Dry, you’ve got three options. Swinub, Vulpix, and Lapras. So this means that you would need a Lapras. Then, you need an Aurorus to get Freeze Dry onto Lapras. It learns Freeze Dry at level 1.
So, in conclusion, you need to do quite a bit of breeding.
Hope this helps! :)