There is a similar question here How do you create Single Sideband Modulation (SSB)? but it does not solve how to recreate the effect with an FM or subtractive synth or a vocoder( if it is possible ). The task is to reproduce the effect of an untuned SSB receiver ( here an example stolen from the question above ). In the reality such a trasmission is done by removing from an AM modulation the carrier frquency and one of the side band, then in the receiver a new carrier is "beated" (ie multiplied) with the signal to reproduce the original audio.
I think the closest effect to use is the so called "Frequency Shift", that mean moving each armonic of the input signal of a certain amount. Different from the Pitch shift that per contrary mantains the armonic relationship. Adding some noise and distorsion can better simulate the radio effect.