If the Virtual Guitarist 2 (VG 2) is not available in Logic (and if it´s also not listed in the AU-manager) please follow this procedure to reactivate the VG2:

• Please delete every VG2 component from your system, exept for the content. Use the Mac OS X search function for this.
• Afterwards reinstall VG2 with the Virtual Guitarist 2 Universal Binary Installer (Only install the downloaded Intel version!)
• Trash the "AudioUnits-cache":
  *user*/library/caches/com.apple.audiounits.cache
• Trash the "Logic-cache":
  *user*/library/caches/Logic/
• Now reboot your system.