It depends on the load torque curve and desired accuracy of RPM.
The load may be inverse, linear , square or cubic with RPM such as pumpers, conveyors, or fans and you did not specify the condition for 900 RPM
Assuming max torque slip is desired target use constant V/f for rated RPM ratio = 900/1390 = 64.7% times 50 Hz = 32.4 Hz and same % Vin.
At reduced speeds VFD and motor assembly, the efficiency is lowest for square and cubic torques loads compared to linear and constant torque loads.
Thus we would avoid using V/F mode at low speeds for pumps and fans with induction types and only use for synchronous types.
Your motor is rated at (1500-1390)/1500 = 7.3% slip (low).
Generally % slip at peak torque is not voltage dependent, but % slip is not the same for maximum power point.
Torque is inverse to applied frequency Constant V/f is used to avoid flux saturation but if too low, there is insufficient coupling to rotor.
Before the peak the torque zone is fairly flat but cooling is reduced so temp. rise is greatly affected.