I've been testing vMix with two Logitech c920 webcams and so far, it seems to work. There is a limitation, though, with multiple c920's; you cannot change the camera settings within the Logitech software for both cameras independently; you have the option of switching between the cameras, but changing the settings doesn't work right. What settings you set for one will affect the other. But it doesn't always work. I know, sounds like a lot of hassle. But after some tweaking and various other things that I did at random to make it all work, I'm getting a good picture from both. My plan is to add a third c920. I'm hoping that Logitech realizes the potential of having control over multiple c920 webcams and updates their software. Here's what I've found so far using c920's with vMix on Windows 8.1:

The cameras like to run in 1080p, not 720p or lower. I couldn't figure out why my cameras were getting out of sync with each other and then I switched them both to 1080p in vMix and viola; problem solved. Why did it work? No idea.

Putting both cameras on the same USB 2.0 bus yielded some frame rate issues. I then put them both on the same USB 3.0 bus and for reasons I cannot explain, I'm getting a solid 30 fps out of both cameras at 1080p. People advise you to put each camera on a separate USB bus, but if you have available 3.0 ports, give that a try. Some have had success on 3.0, others haven't. So far, it's a winner for me.

If you have the capability, upgrade your CPU to an Intel i7; either Sandy/Ivy Bridge (Socket 1155) or Haswell (socket 1150), depending on what motherboard you have. The i7 is much better at video encoding and you'll be glad you have the extra horsepower with four HD webcams, regardless of what brand/model of webcam you use.

I disabled all of the "auto" settings for the cameras. I wasn't too impressed with the settings the software chose.

The biggest thing you need to know is that lack of independent control of each camera's parameters within the Logitech software. I'll be doing more testing when I get a third camera and can let you know what I find. If I can get it all working with three, I'm fairly certain adding one more shouldn't be a problem.

Here's my system specs for reference:

Intel i7 4770k
8 GB 1866mhz RAM
250 GB SSD
1 TB 7200 RPM HDD
3 GB ASUS R9 280x Matrix Platinum GPU
Windows 8.1

If you want to stay in direct contact about the cameras, just PM me your e-mail address.

- Greg

Why are you using webcams if they are all in the same room?

vMix works best with uncompressed inputs. A cheap camcorder will give you uncompressed HD video via HDMI or Component YUV as well as recording compressed H264 to a card.

Brian

I've been using a pair of Microsoft Lifecam Studio webcams at 720p (1280x720) successfully in vMix. I do find that there's more impact to Render time in vMix with the USB cameras than IP streamed cameras, so you'll want to have a powerful graphics card (GPU) if you're going to use more than a couple webcams at one time.