First have a look at peramiter options in the manual.
Looking at another drive, I spotted that the next perameter after the baud rate value was for communication bits.
It stated that 0 was for no error checking and 1 was for stop bit.

Have a read through and see if you get a lightbulb moment.
I admit I know nothing of vfd's.

On another note.
I came across a plugin / driver that said it was needed for Mach3 for HY vfd's to make it modbus compliant. Could this be a possible scenario with linux?.