You can't it is only found in leaf green. .
Where Can You Find Vulpix In Pokemon Fire Red Version For Game Boy Advance?
Go in the mansion in cinnabar island
Either the mansion in cinnabar or go to the grass patch by the row of trainers at least thats how leaf green worked
You can't capture Vulpix in FireRed. Vulpix is one of the few "version specific" pokemon. It IS available for capture in Leaf Green. The rule of thumb is, if you see Growlithe, Vulpix is unavailable and vice versa.
You can, however, trade for one.
You can, however, trade for one.