勇ましい is broader. You can use it to describe a person who isn’t doing anything but merely looks brave. 勇敢 can only describe brave behavior (or someone who has exhibited it). With regard to behavior they’d be basically interchangeable.
勇ましい行動 = 勇敢な行動
勇ましい姿 - the person appears brave (actions need not be part of the calculation)
勇敢な姿 - the person appears brave while actively doing something brave
And I learnt something new as well with “姿”, I didn’t know it could used like that. I guess that’s the difference between learning a word in a “dictionary” versus “in context with real sentences”
I know it’s a little confusing, but my name’s not James. It’s a long story, but it was applied to me by the admins a long time ago, and now it comes back repeatedly even if I choose another title.