Hi RossFive,
Thank you for posting in Microsoft Community.
As both the HDMI connections from your computer and set top box output the video, the option to connect your set top box to Windows 10 computer might not be possible without certain specific hardware.
However, based on my research, certain Vizio TV might support picture-in-picture mode when two input sources are connected, which in your case are two HDMI inputs.
If your TV supports picture-in-picture feature, then you can setup this option in your TV settings.
You may want to check the Vizio support for Picture in Picture configuration. Note that the majority of the configuration is for coaxial setup. You may want to skip that.
You may also want to check the user manual for your Vizio model.
With Windows 10 Movies & TV app, we do have Mini View. However, this only works when you are in Windows 10 environment. You can refer the article Windows 10 Tip: Watch a video while using another app with mini view.
Regards,
Nikhar Khare
Microsoft Community - Moderator