In order to get the chance to join the Darkwraith covenant, you need to make sure you never speak to Kingseeker Frampt after obtaining the Lordvessel. If you do, he'll automatically take you to Firelink altar, where you'll have no choice but to place it on the altar. As long as the Lordvessel is still in your inventory when you defeat the four kings at the bottom of New Londo, you'll meet the other primordial serpent, Darkstalker Kaathe, who will give you the chance to join the covenant if you agree to side with him. If you don't, he'll leave and never return, locking out the covenant for that run of the game.