The letter is strapped to one of the zombie criers that delivered the vampire-hunting message a day prior.
I have not seen you for a long time, if you are the friend I remember from ages past in Whitecrest. I know not how useful I'll be, but I offer my blade to your cause. I've killed all sorts; those of the Carcass Legion, the Cult of the Countless, liches, dragons, and the Predators of the Seas. I will now kill vampires, if you will have me.
Send your response by fish, preferably salmon; it'll find its way to me.