You could try adding this to $HOME/.vnc/xstartup:
However, I'm not entirely sure, whether it disables the KDE screensaver. But something you could try. Add that line as the first command, right after #!/bin/sh-line and before the window manager (like KDE) is launched by VNC.
And if you haven't already done so, I recommend using some light WM for VNC, like IceWM. Takes much, much less memory running in the background than heavy KDE. Just a hint.