It doesn't, it needs a TM.
go suck something
Level 44.
Vulpix learns Flamethrower at level 35.
At level 40 it will want to learn dig
breed it with a bunnary.
Groudon can learn TM28- Dig.
Camerupt can learn Dig by using the TM Dig. It cannot learn Dig by leveling up.
Nincada learns dig at level 45.
Linoone does not learn Dig by leveling up, instead it needs the TM to learn Dig.
If you level it up to level 45 you will be able to let it learn dig, but remember you'll have to delete a move to learn it.
Trapinch can learn Dig by itself at level 29 in Pokemon X and Y. In Pokemon Omega Ruby and Alpha Sapphire, it can learn Dig at level 19.
Vulpix doesn't evolve based on level; use a Fire Stone and it will evolve. Bear in mind, however, that once Vulpix evolves into Ninetales, it won't learn any more moves naturally.