How to Vassalize civs without invading them?

[Caesarea]

I want to "unite" Europe as Germany by vassalizing all the other European civs but whenever I vassalize them by invading them I destabalize them too much and either end up with massive unstable vassal penaltys or they collapse. Ideas?

 

[J422]

Do you give them their cities back once they've capitulated ? Occupied core give instability.

 

[Genghis Khaiser]

You can try with what J422 said. They also may offer themselves voluntarily or through capitulation but you need to have at least twice as much population, territory, and army as them. However, if you are way bigger than them, an excuse to not to be a vassal could be "You've grown too powerful for us".

 

[Alexius08]

However, if you are way bigger than them, an excuse to not to be a vassal could be "You've grown too powerful for us".

That only happens when you have five vassals. The game won't let you have more.