Um, well, it's not as if his character is a HUGE secret. By that logic, even having Vincent and Yuffie in Advent Children is unfair to those who did not get them in the game. To be honest, I don't know why Square left Vincent and Yuffie as secret characters. I personally think they should have been necessary to have in your party. That way, we could have them in FMV sequences.

Out of pure curiosity, why do you 'hate' Vincent? If you enjoy playing FFVII, and have the time, you should go through it again and get Vincent in your party and keep him in there as much as possible. (I know his limits make him a pain in battle, but work with me here)

Also, peek at walkthroughs as you're going through the game to make sure you see all events/flashbacks with him when you get the opportunity. Vincent isn't my favourite character or anything either, but I thought he was a very well thought-out, intriguing, and well-developed/designed character. Maybe you've already done this, and you instead hated Vincent's character for all of this, I don't know.

It's just a thought that one should truly understand something before griping about it, or 'hating' it. (I'm not saying you are, but I'm assuming that if you hate Vincent, you probably wouldn't keep him around in your party, or bother with his sidequests/backstory.)