I believe Vincent and Lucrecia loved one another, but their differing circumstances - namely, his job as her Turk bodyguard - prevented them being together. She was totally devoted to her research, so when she and Hojo got together and he saw a chance to use their child for the experiment, she consented. Vincent tried to stop the abomination which led to Sephiroth's birth, but Hojo shot and wounded him, then performed some alterations on his body and mind before sealing him in that coffin.

Vincent blamed himself for not protecting Lucrecia, the woman he loved, from an experiment which cost her her child and ultimately her Humanity.

She never got to hold her baby, Sephiroth, adn never found out about the monster he became.

We can only hope that Lucrecia was finally able to die once Vincent told her that Sephiroth was dead.