vorpX does not always turn mouse acceleration on. It periodically (every few seconds) backs up the current mouse acceleration state while it is waiting to hook into a game and then sets the acceleration to the last backup value after exiting a game or when exiting vorpX itself.
On a sidenote: disabling the Windows mouse acceleration via the vorpX menu to enhance head tracking for the most part makes only sense in older games. As a rule of thumb, DX11 or 12 games almost certainly use raw input instead of the Windows input pipeline, so the Windows mouse accelaration doesn’t have any effect at all.