If you're still working on perfecting DVIP, I'd like to talk to you about Bosnia. I'm proposing a small change that shouldn't take much time or effort. Here's what I'm talking about.
There is a very contentious debate since the nineteenth century to today over exactly which Yugoslav ethnic groups "owned" exactly which towns and regions, and this debate is nowhere more contentious than in Bosnia. Both Serb and Croat historians and archeologists like to "claim" pre-Ottoman Bosnians as among their own people. Each side has strong evidence in and against its favor. DVIP just splits Bosnia between the culture among what I think is an unrealistic line. Vanilla avoided the problem altogether by having a single Yugoslav culture.
I dislike DVIP's solution, personally, since I think it is ahistoric and buying into a modern issue. I think two alternatives would be better:
First, you could create a separate Bosnian culture and give it to Bosnia. Though the culture was not very much different from either surrounding it, as far as anyone today knows, it has at least as much claim to being an independent culture at this time as Serbian or Croation do from each other. I don't know where you could get space for an extra culture from, unless you have an unused one sitting around still. That, and the fact that a free culture could be better used elsewhere, in my mind make this option inferior.
Second, you could just merge the Serbian and Croation cultures into Serbo-Croatian culture. In the middle ages, Serbs and Croats spoke the same language (as today), had no nationalist sentiments dividing them, and were really only apart religiously and geographically, both of which are already modelled outside of their in-game culture. If all Russian cultures are put together in one, why not Serbo Croation? With this solution, you'd be able to put an extra culture in. Especially if you combine them I think it's best to make the Carantanian/Slovenian culture independent, since in the past and today they are far more different from either Croats and Serbs than Croats and Serbs are from each other, and it's annoying to see at least modern Slovenia as Croation when it never has been. Really all of the duchy of Carinthia should be Carantanian culture in 1066.