I just found out the answer: Joyent turns off the VM CLI for the shared accounts. Their claim is that it would require root access for the CLI to work. See their responses at http://discuss.joyent.com/viewtopic.php?id=25761
But this sounds fishy to me: I can use the VM web interface without being root, right?
This might be solved between Joyent and Virtualmin … it would be good for VM shared users to have the facility.