Vector Control & Slip
The slip is the difference between the mechanical speed of the rotor and the speed of the rotor's magnetic field. The speed of the rotor field and the stator field are always the same regardless of the mechanical speed of the rotor. That is true no matter how the motor is powered or controlled.

It is not true that the stator and rotor flux are always 90 degrees apart, the angle does not exceed 90 degrees, but it is less if the operating torque is less.