http://rreddy.blogspot.com/2009/07/vncviewer-clipboard-operations-like.html
Many times you must have observed that clipboard operations like copy/cut and paste suddenly stops workings with the vncviewer. The main reason for this there is a program called as vncconfig responsible for these clipboard transfers. Some times the program may get closed because of some bug in vnc or some other reasons like you closed that window.
To get those clipboard operations back you need to run the program "vncconfig &".
After this your clipboard actions should work fine with out any problems.
Run "vncconfig &" on the client.