One workaround would be to create a background with two (or more) areas already defined as "borders." In this sample image, the left box is a "border" that fills the whole box. The right box is the same as the left, but it has color bars superimposed on it. The "border boxes" would be the bottom layer, and you would insert layers above it (in this example color bars) that contain your PIP images.

The benefit of having the "border" material fill the whole box is, you won't have to be as accurate in where you put the PIP, as you would if the "border" were just a border.

Does that make sense?

border howto.jpg (25kb) downloaded 5 time(s).

The other option is to create a PNG background with borders already on it, with holes cut out for where your PIPs will go (the transparent holes are represented here by the checkerboard). That becomes your top layer, and then you can resize your pips under that top layer.

border howto2jpg.jpg (49kb) downloaded 12 time(s).

Again, not the solution you were looking for, but a solid workaround, since you can make your background (in my examples just a plain blue) be anything you want, for a very professional look.

This is how I do it, but instead of having solid borders with a solid background, I create the borders alone in After Effects with a CC Light Sweep Effect, render it with alpha Channel for 10 seconds to loop. This way you can have a moving background instead of a solid background. It looks so professional but what I'm saying it would be nice if vMix could do all that in case you want to produce something very quick. Another way to do it in vMix is to have a motion background and place up to 4 OVERLAYS with a border as Picture in picture. I appreciate you trying to help. SAMPLE.png (3,254kb) downloaded 40 time(s).

Here is another example with Picture in Picture overlays
SAMPLE 2.png (3,094kb) downloaded 49 time(s).