Is the extra attack from frenzy on the profile of the Vargheist?
The Vargheist is supposed to have 4 attacks. Its profile states it has 3 attacks naturally, and then you add 1 for the frenzy special rule. 

This is due to the unit being able to lose Frenzy, and as such losing the extra attack, without changing the unit's baseline.

This is the same for other units such as Witch Elves, and dual wielding units, with the extra attack being added to the unit without changing the unit base stats.