Why do you think they must be the same?

You can save multiple vMix Configuration files with different configurations for each show. Also you can save each multi-view setup with Export and then Use Import to swap it for a different configuration if that would work better for your situation. You can even use the Add button on the multiview screen to add a screen arrangement to the presets you can pick from.

Ideally you wouldn't want to reimport settings every time you open up a new project. The multiviewer settings should be saved separately with each project. Its not very wise to have show producers import all the settings every show. You could try to create your own multiviewer in the layers screen, but its no going to have the labels that the regular multiviewer has by default and your going to have to create and position custom labels so your going to run out of layers.