Vassal mechanics have been expanded on compared to vanilla Europa Universalis 4.
The most major difference is that rather than waiting a fixed 10 years before you can start annexing a vassal, there is an individual counter for each vassal before you can do it. This process may be accelerated or slowed down by the state of your country. However, you also no longer have to get your vassal's opinion to 190+ to annex them, just make sure their liberty desire does not rise above 50% during the process.
You are no longer able to enforce your religion on a vassal through a subject interaction. Instead, you only get such a chance when you first create a vassal that has not previously owned any provinces in the game. Otherwise, you will have to send missionaries to your vassal if you want them to follow your religion.
You can interact with your vassals through a decision that will appear if you have at least one subject. From there, you will be able to increase their tribute burden at the cost of extra liberty desire, or reduce it to win their loyalty, or attempt to speed up their assimilation for monarch points.
Assimilation factors
Positive:
-High Centralization (-5 speeds it up by 100% while 5 slows it down by 90%)
-Vassal-related policies (One Currency, Full Sovereignity, Vassal Integration by 10% for each)
-Same culture or culture group as overlord (10% or 5%)
-Diplomatic annexation cost modifier (Calculated from Expansion Capacity and any ideas that reduce it)
-Vassal being small
Negative:
Republican or Theocratic government (100% slower for Republics, 500% slower for Theocracies, 333% slower for Monastic Orders)
-Low Centralization
-Differing religion/religious group (50% or 100% slower)
-Having a Japanese government without being at least Kingdom rank (1000% slower)
-Being a subject yourself (1000% slower)
Vassal upkeep
Some countries, depending on their government, may be able to receive extra diplomatic slots for vassals in the same culture group. The amount of diplomatic slots received depends on the country's decentralization, and the slots will only be applied for existing vassals.
Feudal, Irish, Elective Monarchies: 1 vassal per decentralization point
Indian Monarchies and Medieval Monarchy: 2 vassals per decentralization point
Japanese Monarchies: 3 vassals per decentralization point