Just trying to learn the game.

Bran's ability (from the Core Set). You use this after all plot cards are revealed , but then does the new card override the old one, meaning the old one is ignored. Is this done before the ability on the plot card is read out and its "when revealed" ability actioned?

Vigilant - This ability works even if the card wasnt involved in the challenge?

I have a card in play which has a power in the Marshalling Phase (I think it was Sallador Saan from an expansion). If I am not the first player, can I use this ability before the player actually marshalls any cards?

You resolve the abilities of the plot cards, you don't ignore them. Then you can use Bran to reveal a new plot card and resolve it as well.

Yes, vigilant works even if they are not in the challenge.

The first player gets the first opportunity to make a player action, so you will have to wait until its your turn to make a player action to use Saan's ability.

Ok, so, first player gets the first player action in the Marshalling Phase. Lets say they play a character. That resolves. Then it is my turn next - I can now use the ability on Mr.Saan? So if he didnt use the location as his first action, I can then remove it before he gets to use it.

Paul Grogan said:

Yes. Each player can execute one player action at a time, beginning with the first player, and the opportunity for a player action passes clockwise until all players consecutively pass. In practice, you usually don't make all the passing from the players explicit, it is implicit in that for the most part, the active player will do his marshalling actions undisturbed. Just be aware that after any action, all other players have an opportunity for an action before you do your next one.

After the player in question performs his first action (whether that be playing a character from his hand or triggering a card effect) in the marshalling phase, you can use Sallador's ability before he has the opportunity to execute another action.