First, press Ctrl + Alt + o (I don't why they put Ctrl and Alt in backward order and I don't know why they wrote a capital letter O, which would imply also pressing the Shift key, but the shortcut doesn't work if you also press the Shift key ). Then, head over to the Settings tab and uncheckmark the Options Shortcut Reminder checkbox. It is possible to vassalize the Master, but not the Master's Vassal (as long as the Vassal doesn't break away). The Master gets an inflated self-rating of its Power just by having a Vassal. I played around in a test game and I confirmed that I could get a Master to capitulate to me. From what I can tell, there are some pretty high requirements to make it happen. Someone who can do some code-digging can probably give us the exact answers, but it looks like you need a prohibitively-high relative Power ratio, and other factors seem to be involved, such as your relative total population to that of the Master. My general advise when facing off against a Master + Vassal relationship is just to focus fully on either the Master or the Vassal and eliminate that player completely, thereby explicitly breaking the Master + Vassal relationship and making it a lot easier to capitulate the remaining player. If you choose to focus on the Master, then it's possible that the Vassal will break free, but it might also just be easier to directly eliminate the weaker Vassal.