So you're SSH'ing into the box - are you able to run commands like 'ls' and 'ps' ?
After that, you open VNCViewer. Are you following
this procedure? Or are you attempting to do something like open it from the remote host? Also, what is the error it posts?
Then, after that, your previously established SSH session is terminated by the server, is that correct? Are you able to open a new SSH session into the remote device?