There is provided a vehicle door system for a door of a road vehicle, for example an open-top or convertible vehicle. The door includes a first hinged region thereof and a second openable region thereof. Moreover, the assembly comprises a reinforcement member for inclusion within the door substantially from the hinged region thereof to the second openable region thereof. Furthermore, the assembly further includes a first retention mechanism at the first region and a second retention mechanism at the second region, the mechanisms being resilient in operation to impact or crash forces applied to the door to resist lateral movement of the door when the door is in a closed state.