You CAN use an audio interface with a stock Virtual Studio device, but you have to take the analog output from the audio interface and feed it into the HiFiBerry audio input. You cannot use the USB interface of the audio interface, BUT you will need to plug the USB interface in, so that the audio interface gets power. You will also need to plug your headphones into the HiFiBerry output, rather than the audio interface.
Alternatively, buy another Raspberry Pi (without the HiFiBerry) and use it instead of the stock Virtual Studio device (which you can sell/give to another user). It will also work to disconnect the HiFiBerry from the Raspberry Pi, but this requires taking the Virtual Studio device apart.
We purposely set up the software to use the HiFiBerry if it detects it, because the HiFiBerry interface actually has a little less latency than an audio interface that has to use the USB interface. So you get the best audio and lowest latency if you use the HiFiBerry card to do the analog to digital conversion.