Lots of questions goes unanswered in these forums unfortunately, time to fix that!

I've got a 1920 by 1200 panel myself, and by standard vMix actually stretches the image, however there's an easy workaround to it;

Either you could set your panel to 1080 if it supports maintaining aspect-ratio, like my Dell panel, or you could keep it in 1200 with these settings from the main setting-screen;



This would, however, create transparent bars above and beneath showing your desktop background or any application laying under, but if you just have no applications there and set the background to black (maybe by using http://www.displayfusion.com/ if you'd like to keep your wallpaper on the main screen) you'd be good to go.