If everyone needs a super-logical answer here, assume that Durkon telepathically commands the cleric to say that he's not feeling well due to the stress of seeing a vampire and is going home to lay down, where he stays for the next few days. But that's not going to be addressed in the strip, because it doesn't need to be.
When the domination wears off, the gnome cleric is overwhelmed with feelings of despair at his own helplessness in the situation. Rather than returning to the temple, he spirals into depression, sitting on his tiny gnome couch watching reality programming on his crystal ball and eating tubs of chocolate chip mint ice cream. Every day, he considers getting up and telling his clerical associates what happened to him, but how does he start? Would they even believe him? He wants to hope so, but he never really felt like he belonged there. He was always more interested in the old books and forgotten lore than the rest, who seemed so at ease tending to the parishioners' needs. Eventually, he decides to leave the clergy quietly, accepting a job as an assistant librarian elsewhere. He's constantly hounded by his failure to shake off the vampire's gaze whenever he walks by the church, though, so when his new boss mentions an exchange position with the Great Library of Kurodin that I just made up right in this sentence, he puts his name in to be considered. To his surprise, he's accepted and packs his bags—leaving behind anything that would remind him of his old clerical life. At the Great Library, he finds new purpose and feels energized for the first time since his he was young, before his mother pushed him into the clerical life to follow in his grandfather's footsteps. With a new zeal for research, he makes many important scholarly discoveries concerning the Lu-Ching dynasty of the ancient Southern Lands that I also just made up and has his work published in all the most prestigious scholarly journals. As his academic star rises, his newfound confidence allows him to finally ask out that cute gnome researcher in the Linguistics department, and the two are eventually married in a charming ceremony where he even invites his old clerical friends. Over drinks at the reception, he finally tells them why he left the church all those years ago, and they reflect on that fateful encounter with a vampire dwarf and wonder whether his human friend was ever able to save him.
Either that, or the Snarl destroys the world in a week or two.
Utterly Dwarfed, the sixth compilation of The Order of the Stick is now in stock. Order at Ookoodook (paper copies) or Gumroad (digital PDFs), or pick it up from your local gaming store.