Deceiving the Herd (DLC1RV03) can pick Raven Rock as a quest target. I'm well aware that many radiant quests choosing locations on Solstheim is a deliberate design decision by the developers. In this case though, it doesn't make a whole lot of sense given that the Dawnguard presumably only operate in Skyrim. In fact, I don't recall ever seeing a vampire on the island. Feran Sadri even mentions that the murder target should ideally be high-profile such as someone from the Jarl's court