I hope they don't eat you alive for that ;-).
I know that's not exactly what you're asking for, but there are two otherpossibilities (I think both need the VirtualBox guest additions inside the
virtual machine):
• Let the Windows auto-resize according to the size of the VirtualBox window.
That way, the Window desktop would be slightly smaller, but nothing would be
missing.
• There's some kind of seamless mode in VirtualBox. It then just takes the
windows inside the virtual machine and tries to present them as stand-alone
windows in your WM, removing all the Windows desktop and taskbar, etc.
It's not VNC as such, but I use xmonad through ssh X11 forwarding (so I have
local X server, remote xmonad and all the programs). It works well this way and
in mixed mode too (some programs local, some remote, xmonad on either computer).
I guess VNC wouldn't make much difference, since it's only transferring the
final image somewhere else, so it shouldn't make a difference with the window
manager used.