The External carrier setting might be turned on. In this setting, the VT-4 generates the vocoder sound by using the carrier signal through USB. As a result, pressing the [VOCODER] button does not produce sound in this mode. To apply the vocoder effect, send the sound of an external synthesizer from the computer connected to the VT-4 via USB, while talking with your voice into the microphone.
* Even when using the carrier signal via USB, the tone may differ depending on the vocoder type.