Sam and Dean were genetically engineered by Heaven to be the vessels. I believe the fact that Sam was part of Azazel's plan was more of a coincidence. Cas even says he knows what he did to Sam, but not why. I think it was more that Azazel was trying to make a power play and ended up failing when he inadvertently got in the way of the Winchester brothers.
Really that whole plot gave us two things: the beginning of the show and the reason why John got into hunting, and Sam's psychic abilities (now whether those were always "in" him and just heightened by demon blood or simply because of it I'm not sure)
If Azazel was just trying to create a vessel for Lucifer, Sam wouldn't have been nearly as important. It needed to be the blood line of Mary and John, not the psychic stuff (also a note since Dean obviously did not need any outside force to be a vessel).