I started a game playing as Timeon Dayne of the Torrentine at the Blackfyre Rebellion start-date. I assumed I would be able to choose my side and not be forced to submit to whatever choice my liege has made, however this is not the case. In other times I have found I have been given the choice to join with a different side than my liege's but it seems arbitrary and not properly defined when the choice is given. Playing as Wyman Manderly at the A Feast for Crows start-date I have sometimes been given the choice to join Stannis and sometimes not, seemingly completely at random, which is bewildering. I would prefer if in the next version vassals were given more choice in who they place their allegiance with and not just forced to go along with their liege, or at least it made more clear exactly when it is allowed for a vassal to join a different side and when it isn't. It definitely doesn't fit with the lore that vassals to vassals of the King can't side with the Blackfyres instead of the Targaryens considering many did. Oddly enough, if I, using the console, make myself independent as Timeon Dayne I am able to join the Targaryen side again but not the Blackfyre. Presumably this is due to Targaryen being my de jure liege but even if I give my duchy to Blackfyre and make myself independent again I still can't choose to side with him.