Can soft starter replace VFD?

A soft starter is in the circuit for so short a time, it is not likely that the fluting is coming from the variable frequency drive. My logic is that fluting is a low current long time event. Bearing damage that could occur from the very short and very infrequent duration of starting would have to be a very high energy (for that short time), and would more likely be pitting.

Some shaft couplers are insulating; and therefore, won't drain shaft voltages.

However, all of the soft starters that I have used are line (mains) frequency phase angle modulating. Hence they act as three phase variacs (variable autotransformers). I have not run across any stray voltage problems with these units. However, some soft starters modulate only two of the three phases. I don't know what this will cause.

Regarding VFD's, three steps are needed to protect the motor: 1) High enough winding voltage withstand voltage (dielectric strength), 2) Adequate thermal capability to counter the extra (5% or so) winding heading due to the harmonics, and 3) protecting the bearings from developed stray voltage (grounding, bypassing or insulating).

Once a motor is started (softly) there is no reason to have the starter in the running circuit, so it is "removed" by a bypass (shorting) contactor. If the soft starter is used for running too, then it isn't a "starter".