This is from the original short story this trilogy started from. A bit of it was changed, but not much.
From Protector of the Grey House:
Fifty years after Vincent’s change, Joseph had been ordered to kill him by their benefactor. According to Gautier, young Vincent was too flamboyant and public with his abilities. To weaken him they trapped Vincent in his room. After five days Joseph came to him offering false friendship and staked him. When Joseph stumbled into him, Vincent felt the wood pierce the skin and part his muscles. Vincent did not turn to dust.
When Joseph stepped back, Vincent let the wooden stake dangle from his breast. He smiled languidly, held Joseph’s stare for a full minute and slowly withdrew the stake. Joseph fell to his knees, begging for forgiveness and pleading for his life. Vincent smiled, forgave him, and gave back the stake with the understanding that it was to be used on their benefactor. Joseph readily agreed and Gautier was dead within hours. Joseph had been by Vincent’s side ever since.