Use a different language …
Sorry, sounds flippant but unfortunately VBS MsgBox is an implementation of the underlying Windows MessageBox API call which displays a modal dialog. And a modal dialog/window behaves exactly as you describe, by design. And it isn't something you can change in vbScript (in other languages you can effect a workaround by either a) calling MessageBox API directly with a few of the parameters tweaked or b) displaying a window you create that appears like a MsgBox, but doesn't have the modal behaviour)