If you have a gamepad connected to your PC, please disable it in the Fallout 3 options menu. The game only allows either mouse/keyboard or gamepad input. Not both at the same time.
vorpX overrides the gamepad before running the DirectVR scan since in that case it emulates a mouse for head tracking. After the scan that isn’t necessary anymore, so the gamepad override the gets disabled. When that happens while you have the gampad enabled in the Fallout 3 options, the game disables the mouse like it would without vorpX.
TLDR: After running the DirectVR scan Fallout 3 behaves like it would without vorpX, so you have to disable the gamepad in the game options if you want to play with mouse and keyboard.