There is no way to create a virtual disk with PGP Command Line, or with command line functons for any of the other PGP products (i.e. PGP Netshare Command Line, PGPWDE Command Line).
You can create a virtual disk with Symantec Encryption Desktop (SED) and assign it a mount point (drive letter). The user's guide for SED can be found here:
http://www.symantec.com/docs/DOC7060