I have to say, I don't really care much for the diplo point cost of vassal annexing. I'm not opposed to there being some cost to diplo annexing a vassal, but the 15 diplo/BT seems very high. Even though the cost of annexing is higher as a base, it can easily be reduced. Just having a claim reduces it by 25%. If you go Admin, that's another 25%. There are also several nations that reduce coring cost as part of their national ideas. My primary gripe is that there appears to be no way to reduce the diplo cost of integration. This is preliminary, of course, because they may well add an idea or something to reduce the cost. We just don't know yet.
A solution presents itself, however, in another stat which renders itself obsolete by this new mechanic. Diplomatic reputation. DR currently is a very important stat, as the means to reduce vassal annexing time. It pushes statesman up to the top of the list of desirable advisors. I would humbly suggest letting DR serve as a counterbalance to the diplo coring cost by letting points in DR reduce the cost of Diplo-annexing vassals. You could rework the numbers so that there is always some cost. For example, if you have diplomacy, expansion, and a statesman, it maybe costs like 5 diplo/BT. If your lucky enough to tack on the good diplomats event, its maybe 3. This seems to serve what everyone wants. Devs get a slower early game, because of the cost of annexing vassals, but it accelerates into the endgame, as people flesh out their ideas. you have to make conscious choices about your idea groups, and you can prioritize, and play, however you want.
I think this is an elegant solution. In my personal games, I'm pretty consistently starved for diplo points from peace deals. I actually already rate diplo points as more important than admin. With no scaling coring time anymore, some of this load can more effectively be shifted over to the admin side of the equation through direct coring. It doesn't really make sense to keep diplo annexing free, as this would, in the end, probably make WC to easy. There should be a cost, I just think its a bit high as stated.