At what points do vassals become too big

europa-universalis-iv

I am playing as Venice, and I have Serbia and the Byzantines as my vassals. I am in the process of annexing Serbia, but it is a slow process. At the same time, I want to wage war on the Ottomans, and make them return provinces to Byzantines. However, I am a bit worried that they will become too big to be a vassal, and rebel or declare independence.

Here's how my realm currently looks – I am not sure how much is relevant.

COUNTRY    PROVINCES    FORCELIMIT    NAVAL LIMIT    INCOME    TECH    BASE TAX
Venice        17            44             60          44      9/6/7      114
Byzantium      6             9             20           8      7/7/7       24
Serbia         5             8              5           4      6/7/8       16

If we are looking at size (number of provinces), they are currently 0.35 times my size. I am planning to return 4 provinces to the Byzantines. That means they will be 0.59 times my size, which sounds worrying. If I wait untill I've successfully annexed Serbia, they will only be 0.45 my size (if no other provinces are gained or lost).

So, which numbers are relevant in the equation? Is there a set limit where they will start rebelling? I can see they already have a CB, but so does Serbia. Or will they simply declare independence and that's it?

EDIT: After doing some research, I've found out that it might have something to do with base tax, so I've added that to the table. I believe the magical limit may be 60%, but I am not sure. If that is the case, the math is as follows:

Current: 14%
Future: 43%
+ Serbia: 38%

Is this correct?

Best Answer

Base tax is a factor in Vassalising a nation in the first place. So if a nation has a relatively high base tax it will have a negative effect on your attempts to Vassalise them.

This can vary from +30 to -100 and is calculated by:

60*(your base tax)/(target's base tax)^2 - 90

If the nation is already your Vassal which from what you describe, they are, then the base tax shouldn't be a problem. It shouldn't factor in to whether the Vassal is disloyal or not. I think this instead relies on similar factors to say a Personal Union claiming independence.

I.e. Low relations with the vassal or low prestige

Edit to include info from comments below:

Other factors that effect a nations ability to Diplo-Vassalise another nation (but do not effect a Vassals opinion of their Overlord)

  • Negative opinion modifier if their army is larger than your own
  • You can not vassalise a nation who's base tax is higher than 40