When you say base frequency, is this for PWM or Mach kernel? I assume the former, it isn't clear from the manual that you can set it that high - they recommend 5 - 10 Hz. But of course setting it to 1500 Hz would solve the problem with the smoothing circuit time constant, though reducing the number of speeds steps available.
As you pointed out in a previous post, an emitter follower is not an ideal buffer since it gives a -0.6 volt offset between input and output, which means that when the speed setting in Mach gives a smoothed voltage of 0.6v (6% of maximum, 180 rpm?) the VFD will see zero volts and give zero rpm; and it will also cut 180 rpm off the maximum speed. An op-amp voltage follower would be better - but then the problem is that opamps often can't swing rail to rail anyway. The CA3140 can swing to the negative rail if supplied from a single rail but only to about 2v below the positive rail. So you would need both the opamp connected as a voltage follower and an auxiliary power supply giving say +12v.
Overall, if the VFD input impedance is very high, you may be better off not having any sort of buffer at all but just connecting the smoothing cap straight to the VFD control input.