Why would Elle forgive Vivian, but not Warner?

Firstly, Elle had rearranged her whole life to chase Warner because she believed her happiness depended on him being her boyfriend. By the end she had realised this was not the case, and so chose not to be with him when the offer was there.

Secondly, though Vivian had been snooty towards her at the start, that's all she'd really done to Elle personally. Warner had dumped her for not being serious enough, but then come crawling back when he realised she might be successful as a lawyer. Why would she want to be with somebody who was only interested in her when she was successful in a field he considered worthy? He clearly wasn't interested in her for her. Besides, when Warner tried to get back with Elle, he was supposed to be with Vivian. He was basically saying Vivian wasn't good enough. The same way he'd told Elle she wasn't good enough earlier in the film. Elle was better than that, and she knew it now. The real revelation of what sort of person Warner was came when they were discussing the case, and he made comments saying that she should only look after herself, not her clients. Vivian saw what sort of a person he was then too.

Thirdly, Vivian had shown humility towards Elle, and acted as her if earlier actions were wrong. Warner never really did that.

All in all Elle was better off without him.

I too thought about that. Onething Warner stereotyped her as dumb blonde and not looked her intelligence. He is not right for her. He cares only for himself! He just want the smartest girl. It is all about him. Another, she has now developed a bond with Emmett who really believed in her. On the other hand Vivian appreciated her being trustful. You do become friends with the enemies. It is common and past things are good memories. What Vivian did isn't as bad as Warner.

I think it comes down to integrity. Warner was a narcissist who treated both girls like old/new supply. Who knows what exactly he told Vivian for her to act the way she did to Elle. But as the story progresses, you see the two characters for who they really are. Vivian realized that she was nothing but a 'woman to bring coffee' and saw that maybe she wasn't treating Elle fairly. She warmed up to Elle and they slowly forgave each other's perceptions of each other. Warner, on the other hand, never changed. He only cared about himself, his status, and his reputation. When he told Elle that she should care more about her career than Brook's trust, Elle realized Warner was not a good person. So, in the end, Elle realized that Vivian was a person of integrity and Warner was only after status and ideals.

And that's not saying she DIDN'T forgive Warner; she acted civilly to him. But she realized that he wasn't the right person for her. She, herself, was chasing an ideal and a dream and realized that she had found a NEW dream that was better than Warner.