Help me understand what you are experiencing. When you put your camera and lens in portrait mode and you tighten the head. Does the camera start to move in a downward position while the plate stays in one place or does the entire head start to creep downward?
From the picture of the plate, it appears to have a smooth surface without a lip to fit the underside edge of the camera. This could potentially allow the camera to creep on the plate if the camera has a nose heavy lens like the 70-300 or the 18-200 when zoomed out. Unless Vanguard makes a plate designed for your camera that works with this head you may not have a solution to this issue unless you change heads.