The Crystal Cave area where you encounter Lucrecia, Vincent’s wife, is an optional side quest area of the game that you can explore. The cave is located in the center of the western continent but you can only reach the cave once you have obtained the Shinra Sub.
You can obtain a number of key items for Vincent in this cave including:
Check out the Lucrecia’s Crystal Cave section of the game for more information on how and when you can access the cave:
The map below shows the location for Lucrecia’s Crystal Cave on the World Map:
Chaos
Death Penalty