Damage Reduction + Vulnerability

[Varyon]

Originally Posted by GURPS Characters, p 161

You take extra damage from a particular attack form. Whenever this type of attack hits you, the GM applies a special wounding multiplier to damage that penetrates your DR. Regular wounding multipliers (for cutting, impaling, etc.) further multiply the damage.

(emphasis added)

This is from the description of Vulnerability, and I think answers the question pretty definitively. As already noted, you can take a Limitation on your DR to not apply against your Vulnerability if that's appropriate for your character concept (the Limitation is Bane, from GURPS Horror).