Citando o manual do Virtual Box:
link
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"