Against popular believe, the quest is not scaled. The foes you face are always the same. The reason this "myth" exist is cause the mechanics of the quest changed. Nothing is scaled but the temporary vampire skills. on top of that, the foes got downgraded a good amount too.
I did it at level 14, I was stupid and removed the vampire skills (and don't do this, you can't get them back) but still was capable of finishing it with some trouble.
So my guess is that you can do it when you are level 8.
BUT...why would you??
The vampire skillline levels on XP. the amount of XP gained at low level is so low that the moments of evolution will not come much quicker, if you wait a few levels. So my recommendation is to wait till you are level 15-ish atleast. don't hurry, cause there is no need!