The thing with Lucrecia is that Hojo got her into the experiment and Vincent thought that she was happy while witnessing Lucrecia and Hojo kissing in Nibelheim. Then if memory serves correctly Vincent sees that Lucrecia is deteriorating because of the Jenova cells and confronts Hojo who shoots him and turns him into what he is today. Then Vincent, believing that he could have done something to stop her punishes himself and exiles himself in to a torturing sleep where we find him in the game.
Does anybody know the deal with Nibelheim being rebuilt? I haven't beat the game just yet but I'm in disk 3 and am just training to beat Emerald and Ruby weapon before going into the final dungeon. I'm just confused on when Nibelheim was rebuilt and how that whole thing worked out.
Aeris and Jenova are only connected by about 3 degrees of separation unless you have her fight Jenova on the cargo ship. That's all. Jenova is a calamity from the skies, an alien life form like Dragoon King said.