I found myself trapped within a Vinagre Remote Desktop Viewer VNC session to a remote Linux server console. Trying all keys to find the magic key to pop mouse control back to my local Linux system, finally I found it: the F10 key! In the mean time, I had found the key to make the Vinagre on-line help come up, was navigating through it using keyboard only... and what few help suggestions there were, they always told to go to the Vinagre menu to click on something... "no can do, the VNC session has my mouse hostage!" So just remember, if your mouse gets taken hostage, try F10 to get it released back to the local system's use.