I have long wanted to try a USB VGA card as a kind of tech joke, but they are expensive and rare and it is very difficult to find out before buying one what chipset is in it and whether it stands any chance of working with X11. (Last time I checked, there was at least one chipset that was theoretically supported.)
I am not sure what is meant by "software emulation of a video card". If it supports only 1024x768 16bit 60Hz then that is already far more than USB2.0 bandwidth. So it must have a local framebuffer at minimum, and most likely some primitive blitting and/or painting functions. I would call that a real hardware graphics card, albeit one at the wrong end of a high-latency USB connection.
On the RasPi, even if you could find a compatible USB VGA adapter, there is the additional problem that you would lose the video decoding and 3D rendering functionality of the SoC GPU -- its two impressive features.