I'm using vMix 27.0.0.72 (latest version) and trying the new Zoom plugin feature. This afternoon, I installed the plugin and followed the instructions in the vMix tutorial video for this feature. In my vMix production I also had some vMix Calls. When I brought in the first, and subsequent, Zoom calls, I could hear the audio of the callers both in my headphones which are connected to my vMix production laptop, but also through the built-in speakers on the laptop.
I double checked my Audio Output settings in vMix and "none" is selected for Master Output and "Headset" is selected for Headphones ouput.
I tried changing the default Sound settings on my laptop so that the Output was set to a virtual audio cable to see if that would help but it didn't. In fact, even when I "mute" the icon for the speaker in my system tray, the audio from the Zoom guests comes out of the laptop speakers.
There is no problem with any of the vMix callers or any other audio in my production (e.g. pre-recorded videos); all of those are only heard in my headphones and they go to the Master recording.
The other issue is that my Zoom guests and my vMix Call guests (and me) hear double audio coming from the Zoom participants (*even if there is only one Zoom input*) and only the Zoom participants. Everyone (Zoom participants and vMix Call participants) can hear the vMix Call participants without double audio, and can hear my own microphone and other production elements without double audio.
Also, I uninstalled and then reinstalled the Zoom plugin and restarted my laptop. I also created a new vMix preset and the only input in that preset is a Zoom input. I'm having the same issue with the audio coming through both my headphones and my speakers. No problem with my headset mic in the vMix production laptop. The audio goes to the Master and does not come out of my speakers.
What am I doing wrong? Thanks.

why do you think that?

Thank you Martin, and thank you Koinonia, for your replies, both of which I read just now.

Thank you to Ben at vMix Support for much help with troubleshooting this issue and trying to find solutions. Thank you as well very much to Alan Zenreich, for taking a few valuable hours of time this afternoon connecting with me on a Zoom call (both outside and inside of vMix) to help test and troubleshoot.

We finally resolved the issue (I think)!

First, I installed the newest update to vMix (which came out today, I believe, 27.0.0.75, as well as the latest Zoom plugin as Koinonia and Alan both suggested). At that point, the problem persisted. The Zoom caller audio (and only the Zoom caller audio) was coming out of the laptop's speakers. All other program audio was heard in my headphones.

Then, I changed my vMix Audio Output settings (which had been set to "None" for Master and "Headphone (Realtek Audio)" for Headphones both to "Default Windows Playback Device" for each of those two settings.

That seemed to have done the trick! (Note: inside my Windows System Sound Settings, I had already had headphones selected as the Output device prior to the problem occuring).

Martin, to your comment that "if at some point in the past you had a custom speaker device set in the Zoom app, or there is a strange issue with the Windows audio processing, this mute may not have any effect," it's quite possible that I had a custom speaker device in use previously (e.g. virtual audio cables to route audio from Zoom into vMix when I hosted both on the same laptop) -- not my normal workflow with Zoom but I have occasionally done that so perhaps that played into my scenario here.

I'm going to continue testing this week but am sharing my experience here in case others encouter a similar scenario. Again, much gratitude to all for the help!!

I can confirm there are no changes to this in the latest vMix 27 update, so that would appear to be a coincidence.
As already mentioned vMix already mutes the zoom app, but this may not work if Zoom previous had custom audio settings in the past.

Either way, the fix posted above (of adjusting the Windows audio mixer) will solve the problem in all instances.