I'm sure someone else can chime in if I'm wrong about any of this, but from what I've seen the VIP and OGP file format is very similar. They both are essentially a zip archive, with file groups, and a spec file, and I believe an icon file.
You can unzip both programatically, and read the spec file if you want to read information about the package. The spec file is in an INI file format and you should have no problem reading.
The main difference I know of, is the VIP files cannot be created or edited manually. You cannot take an existing VIP, extract it, add files, then rezip it and use it. This is because there is added checksum information telling VIPM that the package has no corruptions. I don't believe the mechanism to recreate this checksum is documented so editing the package after it is built is prohibited. There is no such restriction on OGP. Extract, replace, and zip works just fine. Both can be installed using VIPM.