To unlock VS Mode you must complete the game on any difficultly level, once this has been done a new option will appear. Select it to load choose from 6 characters and 8 levels.

 

Extra Characters/Levels:

 

To get more Characters/Orbital Frames in VS Mode you must complete the game for a second time, you can either start from scratch (the beginning of the game) on any difficultly level, or you can if you have saved just before the final battle with viola then you could reload this game back up and defuse as well as defeat the boss, once you have completed the game for a second time, save the game data (you must save it or else you will not get the extra characters and levels if you switch off the PS2). Now load up VS Mode and you will now have 8 Orbital Frams to choose from and 10 Levels to battle on.