I have got some old sources written in Visual Basic. There are *.bas, *.cls, *.frm and *.vbp files. As I understand,
you need Visual Basic, not Visual Studio to open these files. or you can open it using a normal text editor like sublime text file separately. but if you want to open it like a project then you should use Microsoft Visual Basic 6.0 / 5.0 . There is a portable version too.
If you still want to open .vbp file using visual studio try to use VB tools for Visual Studio. I think it will help you.