M ount and Blade Warband is an open universe game with no specific end. You play as a character of the ancient continent of Calradia and fight your way to become either a king, or a leader, or a bandit or whatever you, wish to achieve. So you can't become king, ie rise to power, from being a vassal in an existing faction no matter how much every other vassal in the kingdom may like you and support you.-The only way to become king is to either take territory from never being a vassal or to renounce your existing vassalage and keeping your territories.
Mount And Blade Warband Become King Of China
There is no way to replace the king with yourself, per se. However, there are two things you can do:1) Continue to take castles and towns and always demand them to be awarded to you. Once of these times the king will refuse. At that point, you will be given a choice to revolt. Do that and all your towns/castles/villages become your own faction and you - the king. Your former king will declare a war on you, but hopefully you are a big boy by now and can handle him:)2) Find the claimant for your king's faction (he will say he is the true king).
Offer to support his claim. You may have to do a couple of quests, I don't quite remember. But the end result is the same - you revolt against the king and your possessions become your own kingdom. The difference from #1 is that it will be much easier to recruit new lords. You will automatically have high 'right to rule' rating and an option in the dialog with the lords (of that faction) to ask them to join your faction.