Pearl Sky by Elizabeth Hunter

It was a friendly visit until a priceless artifact was stolen. Now Ben and Tenzin are up to their necks in secrets that can kill.

It’s not just any holiday season in the immortal world. Elder Zhang’s oldest servant is reaching the century mark and Ben and Tenzin are ready to celebrate, but no sooner do they land on Penglai Island than a mystery falls in their path. A valuable seal has gone missing, and the artifact was a gift from an ancient vampire who just happens to be heading to Penglai for a long-overdue formal visit.
The Elders of Penglai may have asked Ben and Tenzin’s assistance to find the lost treasure, but that doesn’t mean that eight ancient vampires, their immortal children, and all the humans who serve them will suddenly become open books. Penglai Island is secret for a reason, and the heart of that reason may just reveal the motive behind the theft.