As to the breaker, the VFD listing will give you a MAXIMUM breaker size it is listed with, you can install a smaller one if you like, it's just more likely to nuisance trip. VFDs create line side harmonics, harmonics cause additional heating in components, ESPECIALLY thermal sensing elements in circuit breakers. In other words you CAN use a 20A breaker, because the MAXIMUM is 25A, and 20A<25A. Good luck with that though...
Really, the issue here is the initial basis of design was incorrect. You CANNOT use 12 ga wire on a VFD that has an input current of 19.2A, it would violate 430.122, which REQUIRES that line side conductors feeding an inverter be sized at 125% of the inverter input amps (due to the same harmonic heating effects as stated above). So the initial idea of running a VFD of this size from a 20A outlet was flawed from the very beginning. It would require 10ga wire, all the way back to the panelboard. Given the numbers, I'm guessing this is a 1HP VFD set up for 120V input feeding a 1HP 230V 3 phase motor. The max. VFD you can install that can plug into a 20A outlet is 3/4HP. Someone should have investigated that up front.
Now that all said, WILL it end up functioning? Probably. I always like to apply a test of considering what would happen if there were NO VFD: in this case a 1HP 115V motor. That motor would be 16A FLC, the NEC requires that the conductors be sized at 125% there too, so 20A would have been fine and given that, this will PROBLABLY work OK. It's just not going to be Code compliant. But then again, is a plug-in device required to be Code compliant? That's not a clear cut situation.
CAN you apply a UL508A label to it though? Nope, UL508A will have the SAME conductor sizing requirements as the NEC. So that's going to require that the wires be10ga minimum. CAN you put 10ga portable cord into a NEMA 5-20P cord cap? Yes, in most cases. TECHNICALLY, the fact that the wiring in the WALLS to the NEMA 5-20R outlet will likely have 12ga, is the Code compliance conundrum, along with the possibility of a 20A breaker nuisance tripping. so that's something to consider as you see fit.