If on the other hand it is entirely emulation, then outputting to HDMI may not be so expensive, depending on the chip used.
The biggest expense with HDMI is licensing the thing. The original Vega is an ARM SoC, 16 meg of RAM and 64 meg of flash storage. There's no kernel. It's a bare-metal emulator, and it's using almost all of the capacity of the SoC. Vega+ may take a different approach though.