The two main problems that users reported with UltraVNC in Vista were the use of the "Ctrl+Alt+Del" command and permission issues that caused the administrator to disconnect. As of 2011, UltraVNC has released an update available for any UltraVNC customer that fixes these two issues.
The UltraVNC viewer lets administrators view a computer's desktop from anywhere on the network; it's a remote management tool for network administrators. When issues with Vista arise, the administrator is unable to configure the desktop. If you have not installed the latest upgrade, ask the user to log in to the Vista computer with an administrator user name and password to avoid the issues.
Make sure you assign a password to any UltraVNC installation. This blocks any user from connecting to the computer and only allows trusted connections. UltraVNC is a security risk if you do not have a password assigned for remote connections. A password can be set in the UltraVNC preferences window.