When pressing Alt+PrtScr, the programs on the Xvnc side will see this as Shift+Alt+SysRq. This is a bit annoying since by default both KDE and GNOME use Alt+PrtScr to get a screen shot of the current application.
Red Hat had a similar problem they patched manually. Perhaps their bug entry can be helpful:
No longer a problem in the current code.
Our keyboard handling seems to be correct. Running xev in the remote session indicates that Alt+PrintScreen is coming through fine. However, I don't actually get any print screen from GNOME, but this seems to be a GNOME bug: