I think it might be +2 weapons, as I have attacked the vampires in town that are attacking the Shadow Thieves, and have gotten the weapon ineffective on some of the hits (Yoshi, and Jaheira and they both had +1 to hit weapons). These are those named vampiers, and I think I got the same message against the ones in the Frik's cave but can't rember.
A great weapon to use on them is the azurend (spelling is crappy tonight) ax, throw that puppy and watch the undead scream! And it's realitively cheap too.
Do be careful of them level draining you, as it sucks... unless you have a good cleric with you.
Good luck and death to the Vamps! Hail to the Thieves, Baby!
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Drink and be merry
Jimbo
"Renegade, Rebel, and Rogue"