Reducing Speed of Rotor Vane Without Affecting it's Torque

Make it longer.

Close tighter any valves in the discharge path.

Presumably you could have a different fluid running through your rotary vane motor and change the available torque versus speed combination. A compressed air rotary vane motor will have a completely different speed torque curve if water or mineral oil was pumped through it. You could also just simply provide a drag on the rotation, failing bearings as an example.

slowing the vane requires that the angle of the van is at angle to the rotor. speed is controlled by the degree of the differencials angles between rotor and vane. i can't give you info about new techknowolgy

to angle the van eletronically by remote controll. i'm speaking of 75 year old techknowledge had seems to apply to your question.

sorry, my spell check when shopping.

Can you turn it to an angle less than perpendicular to the direction of the wind?

Are you able to express your problem is some detail, that the members do not have to stress their imagination, to figure what the heck you are talking about?

Lacking that, my bet is pitting and cavitation.

The force being the same the speed and torque are inversely propotional...