I’ve actually checked this and can confirm that sending a 10-bit 444 signal when playing back SDR content should not cause any ill effects.
The device will send a 10-bit signal, but the additional bits will be zeros for SDR content. This will be ignored by the receiving device if it follows the spec. If things look different, then it’s possible that your display is performing some dithering.
This has been confirmed by some other users, who state that there is no problem playing SDR content when Vero 4K is sending a 10-bit signal. I believe that others would have noticed colour problems if this was a widespread issue:
However, I can see merit in outputting an 8-bit signal when Rec709 content is being played. Even if just for the fact that you should be able to see how Vero 4K is interpreting the file and what the signal should be.