I have a coder looking into the problems you have mentioned, manily the redirects and how to tell vbulletin that the payment has been made, via this redirect.
He noted like you did that the author of the article will have to set up a custom confirmation page at paypal, so he is reading up on that now. He will be giving me instructions that I will post on my site for authors to do.
The reason I like this Mod in particular is that it has the extra things in it that would need to be modified via phrases, like "vbVersion", "Mod of the month" Which I want changed to "Technology", i.e. Microsoft, Cisco, Novell, Yada Yada, and "Article of the Month" And "Tagged, Downloaded, Installed" modified to "Tagged, Donated or Paid, and Read".
I'm very interested in the custom page you are talking about using vbadvanced or maybe this one as i use it too.
I'll mention it to the coder that is looking into this.
the phrases could be changed problem simply using VB to rewrite them using the renaming system in styles and templates. so thats actually the easy part .