Above the preview section of vmix there are 3 buttons. The first lets you add displays used to aid in color correction, the second lets you display safe area guides, and the last lets you edit the currently previewing input.

You'll want to use the second button. I assume the GRID 3 option is what you want, it will give you 3x3 lines and this will not show up on output.

If there aren't any guidelines that offer what you want there's a way to make your own that's a little more involved.

1. create your guidelines in a image editor using an alpha background
2. load your guideline image as an input in vmix
3. create an input and use the multiview function to load a specific input (there's a way to setup preview as an input, but that's another set of steps if you're interested), load the guidline image in multivew on top of the desired input (a higher number in the list)
4. if you want a larger preview window of this you can use the fullscreen function and the "Outputs/NDI" option to set that input as a fullscreen preview, or even load it into a multiview layout