I am not a vampire, but I must have been bitten by one of the full-moon spawns. I expected the "buff" to expire on its own, but it's still with me.
Is there some way to make it go away, short of traveling to a priest of Arkay and paying?
Nope. But it's cheap. The last time I needed a cure, it was less than 700g. That's 2 mins of pickpocketing. You waste more time waiting for an answer here.
Used to be, the buff could be removed using any purge effect. I don't think that's true anymore. On it's own, the buff only gives you access to a quest, so it's kinda a non-issue.