At what points do vassals become too big?

Solution 1:

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