Subjugation already gives you 50% WS cost, so you can vassalize something twice as large you normally could. I don't think it should work as PU CBs do, being able to vassalize some 1000-dev behemoth makes little sense. (Just like it makes little sense for the PU CB to work the way it does.)Will you fix the Subjugation CB? Sometimes subjugation of country demands over 100% warscore. Also, subjugation generate a lot of AE. Looks like It just needs fixed warscore and AE and then these missions will look great.
UPD: You granted special 1st tier reform for Ayutthaya, but Mandate of Heaven is also special 1st tier reform. Does this not contradict each other?