This information was passed on to me from Mark (Skywalker) on the forum, so please credit him, not me.
To get a Vampire character (one whose been bitten by Lysandra, for example) to be able to use Vampire bite, equip a non-weapon such as a shield or lantern in your right-hand slot. It's the same principle as the 2nd punch or kick in Kung Fu.
Many thanks to Mark for providing this information: his character, who is now a Vampire, is going back out to take on the Mantraps again one-on-one.
Gael Serran Examiner Sports Edition update forthcoming. Stand by, sports fans!
-Sazerac