VVVF or Fluid Coupling?

I don't see any technical issue. With provisions there is not much of a shock load on the motor, in that case fluid coupling has an excellent capacity to absorb/isolate it from the motor.

Otherwise VVVF should not be an issue.

We don't require the option of running the conveyor at half speed now and the variable fill fluid coupling installed has lots of reliability issues

Does it mean that the conveyor is now constant speed? in that case you don't need VVVF either. Even the two speed could have worked well with gear shifter mechanism.

We actually have the fluid coupling for soft starts to pull away a fully loaded conveyor without transmitting peak load torque to the motor on startup. My question was can I substitue the fluid coupling for VVVF to do this?

There is no technical reason as I said you can not do it.

The question is

a) Cost

b) do you need it? your problem is likely to be only starting, then the conveyour is at constant speed or variable speed? If it is constant why not try for a proper starter?

A link here may help.