Can a VTOL jet such as the Harrier enter a Vortex Ring State?
The main problem with VRT is the airflow through the rotor which is disturbed because it is ingesting its own downwash.



The large area of air accelerated downwards keeps being sucked back in to the rotor because the helicopter sinks back into it. In this state the rotor provides very little thrust - it acts as a sort of divide-by-zero state, as the transition between normal flight and windmilling when the air streams upwards.

The flow field of the Harrier is different:


Thrust is produced inside the engine, which is shielded from the effects of aircraft vertical velocity: air is sucked in through the inlet next to the cockpit.
The air stream velocities are higher. Air is accelerated downward in a much smaller area at much higher velocities. There is no fuselage which partially decelerates the airstream, like in a helicopter: the downward flow field is much cleaner.


The defining problem of VRS is the lack of thrust from the rotor at downward velocities, this would not occur with the shrouded jet engine of the Harrier.