OK, she got over Danny partly because she could confide in Vaughn.
He wasn't concsiously flirting with Sydney when he was with Alice, it wasn't like he should seriously be considering a relationship with Syd.
And he is being kind of a jerk, but he is not in the easiest situation.
You have to see it from different points of view, it is not like he is just going to end his marriage because his death girlfriend showed up, there are so many things to consider at the moment of ending a marriage, and he is obviously fighting to stop feeling things for Syd but he can't.
And don't get me wrong I do want him to leave Lauren.
And the Alice thing.... someone said he left her when he had Syd without caring about her feelings, well It seemed to me that Alice was the clingy one, And he obviously didn't break up with her before, 'cause her dad died.
The thing is that Syd is obviously in love with him, as simple as that, She was in love with Danny too, but it is different 'cause it was the fairy tale version of her life, it wasn't completely real, the thing with Vaughn was. And Syd wants Vaughn, and as she is the hero of the whole show she should and is gonna eventually get what she wants.