Successfully complete the indicated task to unlock the corresponding guitar for purchase at The Vault:

Doubleneck: Get a "5 Star" rank on all songs, except the bonus song, on the Expert difficulty.
Get A Grip: Successfully complete Career mode on the Hard difficulty.
Nine Lives (Daredevil): Successfully complete Career mode on the Easy difficulty.
Permanent Vacation: Successfully complete the game on the Medium difficulty.
Rock In A Hard Place: Get a "5 Star" rank on all songs, except the bonus song, on the Medium difficulty.
Toys In The Attic (Rocking Horse): Get a "5 Star" rank on all songs, except the bonus song, on the Easy difficulty.
Walk This Way: Successfully complete Career mode on the Expert difficulty.