Hi
I assume that your .vpp file is a Visual Paradigm Project File in which case I am pretty certain it won't open in video pad.
(But you can try adding it to the media list and see. 
However, if we are talking about a .vpj file - a Video pad project file then as that is basically a list of data and not an actual video file it can't be incorporated into an existing project.
If you save your current project, you can, if the original files are available on your PC (and have the same names and are in the same loacation as they were originally)...load your old project. You will now have to export the resulting timeline to your PC as an avi (or other format).
Now re-open your current project and then load the avi to media list. Now you can add the avi to your current work.
Thats all you do. 
Nat