Remember: if you have a vassal and that vassal attacks you, do not bribe their army when it arrives at your city because the bribe will remove their vassal status.If you wish that city to remain your vassal you will need to defeat their invading army in a battle on your map.

This may occur in those (mostly rare) scenarios where you have conquered a city and it is later scripted to launch an invasion against you.

I had searched the forum history here and could not find this topic discussed anywhere, but since a player recently contacted me privately about it, I wanted to share this.