Unless there is a separte hot lead to the fan you will need to do some rewiring and run a new cable.
I am not sure that the OP is talking about a fan/light unit. It could be a Plain Jane fan with a light over the vanity. Some of those have the wire junction in the light box, particularly if the fan was added later.either a 14/2 or 12/2 cable depending on whether it is a 15 to 20 amp circuit from the switch to fan/light unit.