The first time you enter the cave, you will see a flashback which explains Vincent's past. It will exit you from the cave automatically. Submerge and resurface near Junon, and fight about 10-15 battles. Return the cave, and you'll see something sitting on the altar. It's Vincent's ultimate weapon and final limit break item.

You can see the cutscene on Disc 2, but you can only get the items on Disc 3.