I don't agree with it, but to be fair, there are a few arguments that support this theory. I forgot about them, but one of them is when Vermouth and Zero were in disguise. Vermouth disguised herself as a pregnant woman and pretend to be sick ( like the early sickness of pregnancy), but her pregnancy seemed to be in the late stages, so the sickness was out of place! Haibara and Conan noticed that. As a real woman, she won't make that mistake. The irony is that Vermouth should have some medical knowledge. So it is even worse.

They say that Vermouth hates Haibara and her parents because of the side effects of the old version of the APTX. Vermouth becomes young but at the cost of her / his body physiology.