Condition for max torque
Induction motor torque is best visualized against "slip" (the difference between synchronous speed and actual speed): -




  at what rpm approximately does it produce its max torque?


I assume by maximum torque you mean breakdown torque so the graph above should help. If full-load torque is at 5% slip then breakdown torque will be about 25% slip but the numerical detail depends entirely on the specific motor.


  If the same motor is run with a VFD which is set at output rpm of
  about 20Hz will it produce a significantly different max torque?


This graph shows a motor run at different speeds from a variable frequency drive: -



If you took the time and calculated percentage slip for full-load torque you would see that for the motor above it would be about 5% at a drive frequency of 50 Hz and fall to about 20% for a drive frequency of 20 Hz.

Maximum (breakdown) torque will be slightly more exaggerated at lower speeds. For instance, with a 10 Hz drive frequency, breakdown torque occurs at about 50% slip whereas for a drive frequency of 50 Hz it occurs at about 30% slip.

See this article (entitled Module 6: A.C. Electrical Machines for Hybrid and Electric Vehicles) for picture above and other useful information.