As far as PhotoPost none of that matters. As long as your using a version of photopost that supports vb4 we auto sense what
vb your using.
You would follow whatever upgrade documentation for your forum that vbulletin tells you.
My suggestion is upgrade PhotoPost to version 8 and then upgrade your forum. Once you upgrade your forum to vb4 then you can follow the steps on our upgrade to generate and install the vb4 plugins.