In X-Men Origins: Wolverine,
Wolverine is shot through the head with what appears to be a vibranium bullet by William Stryker. He remarks that the brain will grow back but the memories won't.
So the projectile penetrated the adamantium skull and damaged the brain. Adamantium is an inorganic substance. It will survive anything that would otherwise destroy the rest of Wolverine's body. But if anything (such as vibranium) manages to damage the adamantium, it is not covered by Wolverine's healing abilities.
Does this mean that Wolverine has an Achilles' heel on his forehead? If so, how did he take a bullet to the exact same spot in X-Men: United (at Ice Man's home)?