Vassals shouldn't be taking up any significant number of tags, since they should simply be merged in in nearly all cases. Some of the previous posts in this thread touched on it.
Something like this makes sense: minimal crown authority = sphere of influence, low = vassal, medium = incorporated (with cores) high = incorporated (no cores).
The only problem I have with this is that CK2 high crown authority does not mean that you have complete control over the lands of your vassals. Incorporating them, therefore, doesn't make much sense, especially without cores. The defining features of crown authority are
1) Unable to declare war on each other.
2) Increased minimum levy from vassals.
The first is adequately mimicked with sphere of influence and vassalisation at medium crown authority. The second, when we start to convert for armies, ought to be handled by giving vassals progressively smaller armies (and the leader progressively larger armies) up to maximum crown authority. At maximum crown authority, the CK2 vassal will therefore be a vassal in EU3, with a very small (starting) army relative to the leader nation. In other words, as crown authority increases then the vassals get weaker and the leader gets stronger. This is in line with the behavior of vassals in CK2.
Another (much simpler) option is to merge all vassals in to the independent realm, and then set the centralisation slider based on crown authority. (decentralisation 5 for min, 1 for max). The only problem is that decentralisation 1 lies outside the maximum slider score for the government types that we're likely to use.
The point of a converter is not to attempt to continue the gameplay of the save file. That is what timeline extention mods are for. The point of a converter is to create an alternate starting point for EU3 gameplay. Thus the resulting scenario should have nearly all of the vassals absorbed, because that is how EU3 starts (with France and the HRE being the two major exceptions).
We're not modifying EU3 gameplay mechanics. Therefore, if the vassals are still in tact (and I think I've explained why they shouldn't be) then it's
still going to be an alternate starting point for EU3. Simply because England in EU3 is unified does not mean that
we should force it to be unified on conversion. The dis-unity of a country shouldn't be discarded simply because that country is unified in EU3. The history of the players world, the world that he has helped create, should not be discarded in favor of the history shown at the beginning of an EU3 grand campaign.