Can you attach mulitple bannermen attachments by fullfilling the condition one time? For example you have two bannermen attachments in your hand and you statisfy the condition are you allowed to attach them to two different characters?

Then another question, specific to the Baratheon Banner, it grants vigilant. Does the character it is attached to get to stand after it is attached? I asked because its condition for attachment says you must win a challenge which would allow the vigilant character to stand. This question seems to be mostly a question of timing of the effect.

first question: I'm pretty sure you can. If it was a location and said response win a challenge claim a power you could only trigger the location once, but if you had more then one location that said that you could trigger each one once. Same principal.

second question: no. Vigilant is before response in the timing window so the bannerman would be attached to late.

In general terms (I know, I do this a lot):

1) There is a "one Response per trigger" rule which says that you can only use a particular Response one time when the play restrictions/conditions are met. The trick here is that "a particular Response" is determined by the individual cards they are printed on, not the effect itself. When you win a power challenge, you can trigger as many "after you win a power challenge" Responses as you like, so long as each Response is on a different card (a bit of a generalization, of course - some effects have "1 per challenge" limits and some cards may have 2 separate Responses written on them). Anyway, this is easy to see and understand when we're talking about using one each of a Bannerman, "Support of the Kingdom" and "Seductive Promise" after winning a single power challenge, but maybe a little more confusing when talking about using two separate Bannermen.

All of this is just a long way of saying you can use two separate Banner cards after winning a single challenge precisely because you are using two separate cards.

2) You are correct that it is a question of timing. Vigilant is a passive effect activated by winning a challenge as attacker. The Banner that gives the character Vigilant is a Response to winning a power challenge. Passives are always resolved before Responses. The Response cannot be triggered (giving the character Vigilant) until after all passives have been taken care of. So Banner for the Storm gives the character Vigilant too late to stand the character for the same challenge that lets you trigger the Banner. You'll have to wait until you win your next challenge.