Vilkacis is incorporeal, so it takes no damage from non-magical weapons and only 50% of damage from magical weapons that lack the ghost touch property.
Vilkacis has vulnerability to silver.
Vilkacis are particularly susceptible to silver. A vilkacis struck with a silver weapon takes an additional 2d6 points of damage. If a creature under the effects of a vilkacis’s bestial possession is damaged by a silver weapon, it also takes this extra damage and receives another saving throw to resist the bestial possession. If successful, the possession ends, and the vilkacis is driven into an adjacent square and cannot use its bestial possession ability again until the following day.
Does the additional 2d6 points of damage from the weapon being silver get halved when the vilkacis is incorporeal too?
Unless the weapon has the ghost touch property, yes.
Where can I find this Vilkacis monster that you are discussing? I've looked in all of the following books: Bestiary I,II,II, Bonus Bestiary, and the Revisited PDFs.
It's from the Carrion Crown AP.