The following features are unlocked by completing the corresponding task and then accessing them by pressing Blue at the main setlist to bring up 'The Vault' track list.

Unlock 'Kings and Queens' by Aerosmith:
Complete Career mode.

Unlock 'Joe Perry Guitar Battle' by Joe Perry:
After the 6th tier warm-up acts complete 'Guitar Battle vs. Joe Perry'.

When you have completed the indicated task the correspondng character will be available for purchase in Career mode.

Unlock DMC
Beat 'Walk This Way (Feat. Run-DMC)' on Career Mode on any Difficulty (Thanks to PhilXZ)

Unlock Joe Perry:
In Career mode defeat Joe Perry in a Guitar Battle.

At the 'Main' menu go to 'Options' and then 'Cheats' and select 'Enter New Cheat' and enter the following codes (each combination of notes must be strummed) to unlock the corresponding feature. To disable a cheat press Green at the 'Cheats' menu.

Air Guitar Cheat Code
RY, GR, RY, RY, RB, RB, RB, RB, RB, YB, YB, YO

Hyperspeed Code - Alter the speed at which gems come down the screen.
YO, YO, YO, YO, YO, RY, RY, RY, RY, RB, RB, RB, RB, RB, YB, YO, YO

Performance Mode Code - The HUD is disabled.
GR, GR, RO, RB, GR, GR, RO, RB

Unlock Precision Mode - ou have to be more precise when hitting the notes.
RY, RB, RB, RY, RY, YB, YB, YB, RB, RY, RB, RB, RY, RY, YB, YB, YB, RB

Large Gems Code - The gems will be bigger on the fretboard.
G, R, G, Y, G, B, G, O, G, B, G, Y, G, R, G, GR, RY, GR, YB, GR, BO, GR, YB, GR, RY, GR, GY

No Fail Cheat
GR, B, GR, GY, B, GY, RY, O, RY, GY, Y, GY, GR

Unlock All Songs
RY, GR, GR, RY, RY, GR, RY, RY, GR, GR, RY, RY, GR, RY, RB

Thanks to PhilXZ for helping out with these codes.

If you want the following guitars complete the indicated task and they will become available for purchase at The Vault.

Unlock 'Nine Lives' (Daredevil) Guitar:
On the Easy difficulty setting complete Career mode.

Unlock 'Permanent Vacation' Guitar:
On the Medium difficulty setting complete Career mode.

Unlock 'Get A Grip' Guitar:
On the Hard difficulty setting complete Career mode.

Unlock 'Walk This Way' Guitar:
On the Expert difficulty setting complete Career mode.

Unlock 'Toys In The Attic' (Rocking Horse) Guitar:
On the Easy difficulty setting get a 5 Star rank on ALL songs except the bonus song.

Unlock 'Rock In A Hard Place' Guitar:
On the Medium difficulty setting get a 5 Star rank on ALL songs except the bonus song.

Unlock Doubleneck Guitar:
On the Expert difficulty setting get a 5 Star rank on ALL songs except the bonus song.

When you complete Expert Career this guitar will become available to purchase at 'The Vault'.