I'm just saying that if you have diplo-annex as it is now, with no cost,
This notion that current diplo-annexation is free just needs to go. You are spending 25 diplo points per core return, a diplomatic slot for over 10 years, and a diplomat during the annexation process. As is, the current system has plenty of cost. The problem is that coring currently is worse. The changes to coring are a step in the right direction, but the simultaneous nerf to diplo-annexation isn't balanced. 15*(Total Base Tax) winds up in these scenarios where it is just not feasible to justify having vassals whose base tax is over a value around 35 (Around 525 diplo points worth). Once you go past that you start hitting these massive numbers for prominent vassalization targets:
Guyenne: 40 base tax (600 points)
Styria: 50 base tax (750 points)
Ukraine: 54 base tax (810 points)
Byzantium: 71/77 base tax (Constantinople is either 9 base tax or 15 base tax) (1065/1155 points)
Mamluks: 80 base tax (1200 points)
Burgundy: 148 base tax (2145 points)
And this is just to annex the vassal! This isn't including all the various 25 diplo points you're adding just to return the cores to your vassals. You're giving up several tech levels to obtain, feed, and annex vassals now. There should be incentive to keep vassals around, but making it so annexing vassals is so expensive doesn't make me want to keep vassals around. It just makes me not want to have vassals, especially when vassal AI is currently only good for sieging.