What is the recommended way for starting an ac induction motor with a VFD?
VFDs have various parameters of interest in your application.


Acceleration and deceleration parameters will give a gentle soft start and stop. Several seconds seems appropriate for you application unless you expect sudden changes in airflow. Your PID response time will have to cater for the lag this will cause.
Minimum and maximum frequency are programmable. You might set a minimum of 10 Hz, for example, so that you don't stall the motor.



  I was wondering if starting the motor with a high frequency AC would provide enough torque in order for the motor to start moving.


Most fans have zero load (apart from friction, etc.) at startup and the power goes up, if I remember correctly, with the cube of the fan speed. The result is that there is normally no trouble starting a fan. At half speed it will draw 1/8 power but airflow will be reduced by a square law. (Check these figures out. It's been a while since I had to do any calculations.)