I'm seeing the same problems: with ML (10.8.2) running on a headless MacPro, screen shared to a different MacPro running 1.6.8, the VM windows (tried both Fusion 4.1.4 and 5.0.2) in Unity and Windowed fail to update correctly. When they are moved, they update; when the mouse cursor is moved across them, a square about 64 pixels on a side is updated. Neither is a viable solution. Lion was fine.
When an external monitor is connected to the Mac, then the VM windows update correctly. That's also not really a viable solution.