Are the optoisolators part of the VFD? If so there's nothing else you can do, it ought to be reliable
if the cabling is decent (twisted pair, or shielded, or both).
If the opto coupler is a separate unit, the wires on its outputs may be subject to noise pick up, and capacitance
across the output at the receiving end may help (perhaps 1nF to 10nF).