The only type of disk that should offer to store the information in the keychain should be one that was created with a PGP Keypair. If the disk was created with a passphrase only user, it would not be associate with a specific user account, and would not store the passphrase in the keychain.
The keychain is actually storing the passphrase for the keypair, and not the passphrase for the disk itself, and the keypair unlocks the disk.
If youhave previously encrypted virtual disks that are encrypted to a keypair, they should wor fine if that keypair is loaded into Symantec Encryption Desktop. When the keypair is imported, the first time it is used it should ask if you want to store that passphrase in the keychain.