The Vincent and Lucretia storyline has a lot of unexplored potential. He was a nobody, really - just a bodyguard, a hired gun, the not-very-bright son of a brilliant father. He was in no way Lucretia's intellectual equal. He was always tongue-tied around her, too, probably intimidated as much by her intelligence as her beauty. I imagine she was somewhat older than he was.
I don't really know how to put this into words, but - even though they're closely associated in the game, and even though he was obsessively in love with her, I don't feel as if their tragedies have much to do with each other. Lucretia would have committed her terrible crime (the illusionary crime against Sephiroth) with or without Vincent on the scene. He's kind of irrelevant to her - or at best an added, secondary complication. And without Lucretia, Vincent would have found some other unattainable woman to mope after and destroy himself for, because that was his nature. I feel as if each is trapped in his/her respective tragedies, which are only coincidentally related: he happened to be there when she was committing her crime, and she happened to be there when he was ripe for a disastrous unrequited romance. Both of them are now condemned to live an eternal existence somewhat less than fully human, and still neither of them can actually touch the other, in either a literal or a metaphorical sense.
I can't remember if Lucretia ever said, in DoC, that she loved Vincent and meant it. Did she?