Especially considering that they will usually take, at most 3 and often just 2 provinces before refusing based on overextension, and indeed the coring time vs annexing is trivial.
FYI it is not "at most 3 and often just 2 provinces." Vassals ALWAYS stop accepting land due to overextension at EXACTLY 24% overextension. This means that vassals will accept 6 cumulative base tax and no more. The nice thing about this is they will always accept more land (provided they want it) if they have anything less than 24%.
For example, say you attack the Mamluks and take 6, 1 base tax, provinces. Your vassal, say tripoli, will accept all 6 of them (again provided same culture or same religion). Now suppose, however, you take 5, 1 base tax provinces, and 1, 7 base tax province, such as Cairo. If you feed the vassal the 5 1 base tax provinces first, the vassal will be at 20% overextension and will accept the 7 base tax province. However, if you feed it the 7 base tax province first, they will be at 28% overextension and will therefore not accept any of the 1 base tax provinces.
5 base tax + 14 base tax = OK
6 base tax + 2 base tax = FAIL
1 base tax + 4 base tax + 17 base tax = OK
I hope this is clear.
Plan your land grabs accordingly.