I think Vermouth chose Araide as a disguise target to get close to Haibara (and Ran and Conan), meaning Vermouth already knew Shiho was at Teitan Elementary when she targeted Araide. However, Vermouth didn't attack Haibara immediately because she wanted to separate Shiho from Shinichi and make sure the FBI was neutralized to avoid complications with her plans. That meant taking some time to find out where the FBI lived, coming up with a way to ditch Jodie, and getting a better feel for Haibara's relationship with the others so she can figure put how to separate them.Kudo Shinchi wrote: I don't know if this was ever answered but, in Volume 45, chapter 458 Conan wonders to himself why Vermouth didn't immediately find Haibara out when she disguised as Dr. Araide and began working at Teitan Elementary. Since Vermouth figured out that Sherry had shrunk into a child, all she had to do once she began working at Teitan Elementary was to ask about any recent transfer students and she would have tracked down Haibara immediately, so Conan wonders to himself why didn't she notice Haibara right away?
The Viz translation of Conan's thoughts were this: "Come to think of it, the fake Dr. Araide worked at Teitan Elementary too... if she [Vermouth] suspected that Anita (actually Haibara, Viz changed a few names) had turned into a kid like me, why didn't she notice her at Teitan Elementary? A new transfer student would've been a huge red flag, so why didn't she know?"
Did we ever get an answer to this?
Conan interpreted the lack of Vermouth doing anything (that he knows about) as Vermouth not knowing about Haibara yet when she disguised as Araide, but I think that is an incorrect deduction given what we know about Vermouth and her plans. I'm pretty sure she knew at the party who the two children were when she saw them, which explains her uneasiness and sudden change of plans to stay in Japan. She joined the school when she found them, using her disguise to get close and learn about them.
This explanation resolves all the contradictions, although it requires Conan to be partially wrong about a deduction. (Kind of a red flag...) As far as we know, there is no particularly good reason for Vermouth to work at Teitan other than to target Haibara and Conan. If she was looking for Shinichi and Ran, she could have subbed for a day or just stole some records. (Or the boss is in class 1-A and he was bored.)




