Everyone can vote on any noble player. But the vote will count only if the player they voted on decided to be a throne pretendent.
The question is, should there be a list who is a throne pretendent or should it be secret?
The thing is the current king could try (realisticly speaking will always try) to make their competitors miserable/weak (like stripping from kingdom offices). Or outright call them traitors and punish everyone who is trying to associate with them. On the other hand, having a list who is the throne pretendent would make it easier to vote, also you could then know who got how many votes the last time.
Take into account the medieval setting, so it does not necessarily have to be a democratic election as we perceive it today
![:D](http://public.gamedev.net/public/style_emoticons/<#EMO_DIR#>/biggrin.gif)
There will be also a parallel vassal system (a smaller group of players with one being the top senior), the seniors of each vassal group will get much more voting power (part of the votes of his vassal will be transfered to him). So there will be a clear minority of nobles who get an uproportionally high voting power.