I was attacked by vampires in Whiterun and unknown to me as I used a dismar shout I hit two NPCs one was the partiarch to the Grey-Mains clan. I latter on got a letter from Skulvar Sable-Hilt asking to see him next time I am in Whiterun. I found him and went to talk to him about the job but he tries to sell me a horse and doesn't even mentions the note he sent to me.

Is there are certain place or time I need to be for him to trigger with the correct conversation ?

did reading the letter activate a quest?

He/She'd have to to know who sent it bro.

The quest is bugged. Try dropping the letter and picking it up again.