FFX2

Ahhh thank you!
XD
the storylines are REAAALLY confusing and noboy seems to want to write it anywhere, i remember that i looked on bluelaguna and FF-Fan but i couldnt find anything! ^^ so annoying!

As for DoC;
Vincent Valentine is (im going to copy this from ff7-doc.com)
Vincent Valentine was a Turk and was selected to supervise the science project led by his farther, Grimoire Valentine. He fell in love a scientist who was assisting Grimoire in the project, called Lucrecia Crescent. When Grimoire died Lucrecia left Vincent, because she felt guilty about his father’s death. Vincent later catches up with Lucrecia when observing the Jenova Project based in Nibelheim led by Professor Gast. Here Vincent proposed to Lucrecia but she turned him down and started a relationship with Hojo who was also working on the Jenova Project.

Lucrecia fell pregnant for the baby to become the next experiment, the effects of Jenova Cells on an unborn child. Vincent objected to using living subjects being experimented on, especially Lucrecia but she continued regardless. The experiment made her become increasingly ill and Vincent confronted Hojo about it. Hojo ended the conflict by shooting Vincent in the stomach. Hojo then shut him in a Lifestream Pod in the Mount Nibel Mako Reactor where Lucrecia infected Vincent with contaminated Mako, G Substance, and became Chaos.

Vincent, being influenced by Chaos, filled with rage and broke free of his pod, Lucrecia used some meteria that could control Chaos and put Vincent into a deep sleep. When he woke up on the operating table in the basement of Shin-Ra Mansion but Hojo and Lucrecia had already gone. Vincent was distraught with rage about the modifications to his body and shut himself in the coffin. Hojo locks the coffin and the room, places the key in the safe and hides the combination and sets out the clues to find it again.

Hmmm okay so anyways.
thats Vincents Bio, but basically, in FF7 (the original) he was more of a side character, who you could get by going to Neibleheim in any disc;

there are a bunch of clues set out all of the Mansion there (by hojo) and if you follow them they give you a the combination to a safe in the upstairs room, if you correctly open the safe and defeat the boss inside (Lost Number) you'll get the "Odin" materia, and the "basement key".

Now when you go into the basement, in the purple hallway if drag your analog stick to the left you'll notice that there's a brown wooden door in the purple hallaway that you couldnt open before... the key opens the door and inside theres a coffin (where vincent is) you can pop open the coffin, and Cloud (or Cid if you're in disc two) and Vincent will pop out and complain about how you woke him up from his restful slumber (if you actually do the little sidequest and get vincent then you'll be told that he's been asleep in the coffin for 30 years before cloud and party find him)

If you talk to him abotu sephiroth, he;ll go back into his coffin, where you can wake him up again; if you ask him his name, he'll join your party and as you leave neibleheim mansion he will join you team and you can use him in any battle afterwards.

He doesnt have a huge role in VII because he's a side character (like Yuffie) which is why he doesnt appear in any FMV sequences.
The only real thing he does is explain his past as a turk (if you take him to the waterfall near neibleheim; lucretcia's cave; in disc 2 and 3) and defeat hojo and have a brief discussion on how Hojo should've been the one locked in the coffin for 30 years (if you take him in your party when you verse Hojo in Disc 3)

Now in Advent Children (lets see if i get this right) it's been 5 years since You defeated sephiroth in VII.
So basically the whole gang returns to take out sephiroth and jenova (again) and Vincent explains to cloud what jenova is, and how Kadaj,Loz,and Yazoo came to be formed and how they are connected to sephiroth. He also saves Clouds ass in the Forgotten City, pwwns Bahamut, and has a brief discussion with cloud on whether of not sins could be forgiven.

The only real connection between them is that Cloud awoke Vincent from his slumber, and informed him of how Sephiroth (who was actually the mako-child lucretcia gave birth too) was alvie and trying to take over the world. Vincent pretty much only came along to defeat sephiroth so that he would no longer have to bare the burden of carying his sins around. (he blames himself for sephiroths birth - and for sephiroths insanity- so he belives that by defeating him, His sins will be forgiven and whatnot)

Hope that made sense XD
♥

Fayth: "Yes, you're a dream of the fayth. You, your father, your mother, everyone. All dreams. And if the fayth stop dreaming..."
Tidus: "No! So what if I'm a dream! I... I like being here."
Fayth: "We've been dreaming so long... we're tired."