You can continue without Vincent, he's a completely optional character, but if you do pass him up and wait until disk three to get him, you'll miss part of his ultimate weapon quest(I think it's the ultimate weapon). I know that you have to go to the waterfall where (SPOILER)you meet Vincent's mother with Vincent in your party in disk 2 for some part of the story and goodie that you will recieve when you revisit in disk 3 with Vincent again.