Playing a Qing campaign. Just verified none modifier affects the required mandate to convert a tributary into a vassal, it's just a plain 40% of the current development. At least administrative efficiency should affect it. Other modifiers that might affect it too for the gameplay and consistency purposes:
- Core cost reduction
- Diplomatic reputation
- Annexation cost reduction
- Core cost reduction
- Diplomatic reputation
- Annexation cost reduction
- 6
- 5