How do you know when you've completed the vampire side of the questline? I heard the elder scrolls are weightless once you do, I just killed lord harkon and they still are 20lbs a piece, do I have to totally wipe out the dawngaurd before they are weightless?
Torchbug
The vampire questline in Dawnguard is finished when you kill Harkon, but there are still lots of side quests. See: http://www.uesp.net/wiki/Skyrim:Dawnguard_Quests
However, the only weightless Elder Scrolls I've ever heard of are with PC mod. Some people have reported that ALL quest items, though marked with a weight, are actually weightless, but I haven't bothered to check my inventory because it's never been a problem for me (my Nord vampire assassin has a carry weight of almost 600). You can test this by stowing all of your inventory in a chest (a safe one, in one of your homes) and seeing what your encumbrance says while only carrying quest items.