I was traveling as a vampire from Windhelm to Stony Creek Cave in search of a blood ring for Feran Sadri. I get attacked by several Dawnguard. After looting the bodies I see one has a writ of dawn letter saying that I must be put to death and that they have the full support of the Jarl, which leads me to my question(s):

#1 which jarl? Riften? Being that Ft Dawnguard is so close to Riften.
#2 if that's the case then how come the Jarl doesn't have me arrested or killed upon entering that city?

And I just thought of something as I type this. Is Feran Sadri related to the owner of Sadri's used wares in Windhelm?

Since you're in Eastmarch, Jarl Ulfric approved of it.