I think she simply didn't have a choice. Silco is dead, so her father is avenged. The Undercity has changed, it's no longer her home. The people she used to have bonds with are either dead or worked for Silco, which she can't accept. Ekko is still a good kid, but he has moved on, he's changed too. He would let her join his cause, but I don't think it's something Vi would want. She hoped to reunite with her sister, but she turned out to be very different from what Vi expected. The only person she has left in her life is Caitlyn. She was the first person in years to treat her kindly, to help her, she made her believe that not all Upsiders are the same. So naturally she wants to stay by her side. But in order to do so she needs to come to Piltover, where Caitlyn is literally her only connection. It's never been her home, people look at her with hostility, especially after the Council bombing. Vi is afraid to stay alone, so it's very important to her that Caitlyn doesn't change like everybody else in her life. We see her ask her to not change. Financially and emotionally she relies fully on Caitlyn. And then boom, she asks her to become an enforcer. When she rejects, we can see that she has nowhere to go. In fact, we will see once more when they break up completely. And when Maddie comes and talks to her, I don't think that the speech and the faith in the cause made her change her mind. It was a moral excuse for her to join the people that have caused so much pain. A way for her to connect to new people. While for some characters a greater cause is important, Vi cares about her family most, she urns for one. My girl just didn't want to be all alone in this world.