I was on the vampire side quest where you have to kill the dawnguard person masquerading as a pilgrim. the location for the quest is honeyside in riften but I cant buy the house for some reason. I even did all of the skooma dealer quests. how can I fix this? I am also on the xbox 360 by the way.

Honeyside? It sounds strange. Anyway, you have to become thane in order to buy honeyside. After the skooma quest you have to help random people and then report to the jarl.

Are you following a quest marker? Sometimes a marker into Riften will locate itself on the exterior Honeyside door, instead of a regular gate into the city. I'd say try looking around the town and in the Bee and Barb and see if you can eyeball your victim without a marker. They usually spend some time in an inn and wander around dangerous, shadowy areas of town in the wee hours.