Starting with VirtualBox 4.2.0 there is another extension pack,
VNC, which is open source and replaces the previous
integration of the VNC remote access protocol. This is experimental code,
and will be initially available in the VirtualBox source code package only.
It is to a large portion code contributed by users, and is not supported
in any way by Oracle.
The keyboard handling is severely limited, and only the US keyboard layout works. Other keyboard layouts will have at least some keys which produce the wrong results (often quite surprising effects), and for layouts which have significant differences to the US keyboard layout it is most likely unusable.
It is possible to install both the Oracle VM VirtualBox Extension Pack and VNC, but only one VRDE module can be active at any time. The following command switches to the VNC VRDE module in VNC:
VBoxManage setproperty vrdeextpack VNC
Configuring the remote access works very similarly to VRDP (see Section 7.1, “Remote display (VRDP support)”), with some limitations: VNC does not support specifying several port numbers, and the authentication is done differently. VNC can only deal with password authentication, and there is no option to use password hashes. This leaves no other choice than having a clear-text password in the VM configuration, which can be set with the following command:
VBoxManage modifyvm "VM name" --vrdeproperty VNCPassword=secret
The user is responsible for keeping this password secret, and it should be removed when a VM configuration is passed to another person, for whatever purpose. Some VNC servers claim to have "encrypted" passwords in the configuration. This is not true encryption, it is only concealing the passwords, which is exactly as secure as clear-text passwords.
The following command switches back to VRDP (if installed):
VBoxManage setproperty vrdeextpack "Oracle VM VirtualBox Extension Pack"