In vacuum there is no convection. Only conduction. This means the mount and material used is critical to heat flow. Smaller motors utilize conduction to a larger extent than large motors with lots of surface area that use more convection. So, if it is a small motor <50mm diameter than its derating in vacuum would be less. A large motor >50mm would require a more significant derating. General rule of thumb would be to assume 50% of the heat transfer. This mean that the motor would be derated to 75% of the torque output because power is mainly a function of copper losses that proportional to the square of the current.