[It does sound like manipulation, but Vanya doesn't really know why, either. As more and more of his true nature came to light, Vanya was just so... confused and scared. He was the only one she felt like she could trust until he wasn't, and she continued to anyway, because what else did she have?

Until she couldn't ignore it anymore. Until he became unforgivable. Until he became another monster under the bed.]

Allison told me that.

[Stupid, stupid, stupid. Allison told her all along.]

He had all of dad's old notes. From when we were kids. And he used them to...

[Vanya is starting to feel sick. This is too much, right now. Too much too soon.]

...I don't want to talk about him anymore.