There is not a simple software way of doing this, but there is a way. Wire the output of the variable through a relay before it goes to the fan. (L1 goes to one VAR terminal, the second VAR terminal goes to the relay, and then the second relay ...
In order to control a VFD, you will need 1 0-10v output from a VDC-4, as well as 1 follow relay from a RM-4 which controls the VFD's start/stop contact.