Little is known about Vincent.
The game reveals that he was a Turk sent to guard the ShinRa scientists
working on the Jenova project in Nibelheim. There he fell in love with one of the scientists, Lucrecia.
Sadly, she became involved in an experiment to infuse Jenova cells into a human fetus. This led to her supposed death, and to the birth of Sephiroth. Vincent blames himself for her death because he was unable to stop the experiment; all he could do was watch as Lucrecia suffered and eventually died after Sephiroth’s birth. Consumed with guilt for his own failings, Vincent sleeps in a coffin letting his nightmares punish him for his sins.
The man directly responsible for Lucrecia’s death and for placing Vincent in the coffin, Hojo, remains free. Before Hojo locked Vincent in the coffin, he experimented on Vincent, giving him red eyes and a metallic claw in place of his left arm. The experiments also gave Vincent the strange ability to transform into four different forms. These monstrous forms make up Vincent’s limit breaks during the game.
You first encounter Vincent in the basement of the Shinra Mansion in Nibelheim. There Cloud and the party find Vincent in his coffin, where he has remained for many long years. After learning of Sephiroth’s insanity, Vincent agrees to help Cloud and the others on their journey to stop Sephiroth. However, Vincent’s main goal is to find and kill Hojo so that he may finally avenge Lucrecia.
After Vincent kills Hojo, he continues on with Cloud and the others to save the world from Sephiroth. At some point during this time, he encounters Lucrecia, or her spirit, in a secret crystalline cave behind a waterfall near Nibelheim. The conversation with Lucrecia prompts flashbacks to the Jenova Project and Lucrecia's demise. The flashback leaves one with more questions than answers and has caused much speculation by fans about what really occurred during the Jenova Project.
My speculations about Vincent and the Jenova Project, as well as the dialog from important game events concerning Vincent can be found through the links on the left. I would recommend reading “Awakening Vincent” and “Encountering Lucrecia” before reading my theories on Vincent, since many of my conclusions are drawn from those conversations.