To not get a bounty you need to feed on enemy NPCs. Any enemy that is a human you can feed on. Don't feed on people in towns just go for a short ride into an enemy camp and sneak around and get all your feeding done. To level up your vampire abilities you just need lots of exp. it doesn't matter if you have any of the powers slotted it's based off of total experience NOT how much you use it. Also the vampire quest.(if I remember right) is a lv 37 quest. So the vampire skill tree is operating under the fact that you shouldn't get the ability until lv 37. So if you get it at a much lower lv like 15 then your going to be getting less experience from lv 15 quests and enemy's that is going towards a skill that is assuming your lv 37 so it's going to lv much slower. For a frame of reference I got the ability when I was lv 32 and I didn't max out the skill tree until lv 47 and that was without having any of the abilities on my bar. Let me know if this helped!