Vampire Faction -- Deceiving the Herd
To begin completing the side-quests go to the Alchemy Room and speak with Feran Sadri, selecting the chat option "I heard you might need help" to learn that the Dawnguard have been causing problems for the collective and a setup to draw negative attention to the Dawnguard -- and this flags the quest DECEIVING THE HERD.
You need to select a notable target, murder them in a high profile way, and plant evidence that points to the Dawnguard. To help facilitate this, he provides you with Dawnguard armor to wear when you do it!
Killing a prominent member of the Herd and planting the note
The target needs to be in Whiterun (or whatever city ended up being the capital), and optionally it can be an important person... Wearing the uniform is optional as well but hey, why take half measures?? Note that "important" can mean merchant, bard, or official, but you do need to be sure that there are witnesses to the killing and that you plant the note by using the loot window and "storing" it on the body.
I chose Anoriath, the merchant with the stand by the Inn -- but you can pretty much pick any prominent person in the city. The really strange thing is that as I was making my escape I was stopped by a guard (as you usually are when you have a bounty) and given the opportunity to pay it off. Paying it off did not mess up the quest, how strange is that??
Head back to Feran and report your success -- I changed out of the Dawnguard uniform so as not to freak anyone out before going into the castle heh -- and when you report in he basically gives you an after-action assessment in which he notes what conditions you complied with and then rewards you based on the level of your success in doing what he asked you to.
The reward that I received for completing all of the criteria was a Necklace of Disease Immunity -- which gives 100% Disease Resistance. That completes the quest DECEIVING THE HERD.