When I open VP 18 I get a popup for an update. After I install it, it tells me to shut off the program in order for it to install. When I shut it off, everything goes away and nothing is installed.
Very strange as it should close the open version itself during the upgrade. Maybe contact Magix on this one.
Try downloading from your MAGIX Account or from MAGIX Download Center. First open the Task Manager and kill any VP processes left there.
A repair install 'should' fix that, The same issue occurred on Sound Forge Pro-14. Download the latest build of VP-18 and install it over the top of the exiting one. All your customizations, layouts and such will remain intact.