In the YouTube vMix video of the beta, the Vertical Safe Area lines are demoed. However, it seems that you can only select on option at a time. Since you can run three live streams, all of which can have different dimensions, you should be able to select multiple options. This way you can confirm that what is being streamed fits on screen. For example, one stream might be 16:9 and another could be 1:1. Being able to show safe area lines for both at the same time would insure that everything is correct and eliminate all the extra mouse clicks to check them both.

Just a thought,
Art