Problem
All current Steinberg VST instruments offer several formats for the installation. Next to the VST standard installation one can also install an Audio Units (AU) version for the use with sequencers like Apple Logic. In rare cases the instruments are not selectable after the installation and validation and thus cannot be used.
Cause
As soon as Apple Logic detects a new AU compatible instrument on initialization, the AU manager is started in order to validate the instrument. Depending on the instrument, this process can take up to several minutes to be completed! Whenever this validation is interrupted or an error occurs, the instrument is not approved and won't be available in Logic.
Solution
Please start the Logic AU-Manager from within Logic:

Menu "Logic Pro" -> Preferences -> Start Logic AU manager

Logic will be terminated and the manager starts with a list of all AU components available. The relevant instrument (e.g. Virtual Guitarist 2, Virtual Guitarist 2 FX) should be listed here although it has not passed the initial test. So if you find the entry unchecked and the column "Compatibility" does not show "passed validation" you have to start the test once more ("Rescan"). After this additional (usually successful) validation process, Logic has to be started again and the instruments should finally be selectable.