When streaming from vMix to YouTube I get errors from YouTube - "YouTube is not receiving enough video to maintain smooth streaming. As such, viewers will experience buffering"
This is definitely a vMix issue since I tested sending to YouTube (using the same bitrate etc.) using a Teradek VidiU encoder and there was no problem.

I used the YouTube recommended settings for both tests - 1080p 4.5 Mbps bitrate, keyframe frequency 2 sec., Audio bitrate 128 kbps, Strict CBR

Initially, vMix showed red in the stream button and then quickly turned to flashing yellow and stayed yellow.

YouTube started off with "Excellent Connection" and then gave the error message: "YouTube is not receiving enough video to maintain smooth streaming. As such, viewers will experience buffering" The stream health now showed as Poor.

Another warning then appeared in YouTube: "The stream's current bitrate (1565.56 kbps) is lower than the recommended bitrate of 4500 kbps"

A while later, another error message appeared on YouTube: "Please use Keyframe frequency of 4 sec. or less - currently keyframes are not being sent often enough - the current frequency is 5.8 seconds""

Note: I'd set YouTube to "Normal Latency"

NOTE: The above test was carried out using the FFMPEG2 and the FFMPEG Applications which gave the errors.
NOTE: Using the FMLE application showed no errors and a Normal stream - solid red and YouTube "Excellent Connection".

So vMix was sending a faulty stream when using both the FFMPEG applications, despite the vMix stream settings set to the correct YouTube recommendations of: 4.5 Mbps bitrate, keyframe frequency 2 sec.

Maybe this is something vMix should look at and provide a quick fix for?

P.S. Sent vMix a Support Report

The problem seems to be solved by using FMLE application in the streaming settings instead of any of the FFMPEG applications.