I think I found the issue. It was due to the fact that CABLE Input was selected as the playback device, instead of Speakers.
This may be worth adding to vMix documentation to help other users who may run into the same situation, because the default device automatically changed to CABLE Input during the installation of VB Audio Cable driver which the users may not have realized.

The other important thing I noticed is that vMix Desktop Capture seems to remember the default device when the Desktop Capture program started, and keeps that setting even if the user changes the playback device to something else (like Speakers). This behavior would make it hard to troubleshoot issues like mine, as users may try changing playback devices without realizing Desktop Capture also needs to be restarted to reflect the playback device change.