The F-35 and the harrier have a bug that the only way to engage the VTOL mode is to start with a normal takeoff before the vectoring works properly. done exactly as it asks but still requires the vehicle to move forward first.
We were looking into this last week. The solution is to move them to "airplanex" simulation (PhysX) instead of "airplane" (old BI physics). Then vectoring modes work as intended. BI was also notified they don't work properly in old physics engine. Changing it to "airplanex" is not an easy job though, we ran into a problem of wheel brakes not working at all and couldn't figure it out yet.