Yeah Ewan's performance is just fantastic. So much nuance on his face when he tells Tala "It's just Ben now" and his cries in the fire are just heartbreaking.
As for him running from Vader, yes it's fun to have a good chuckle about that here but I don't have a problem with it for three reasons:
1. He just realized a few moments earlier that all Vader needs to kill you is line of sight!
2. If he died then Leia was most likely screwed and since a duel was
at best a 50/50 coin toss that he'd lose he didn't want to risk those odds for her sake.
3. He's clearly still tormented about Anakin and I'm not convinced that he would have wanted to
win that duel, since it'd require him doing what he never wanted to do in the first place--kill Anakin.