Okay I'm just pumped that we finally got a Kaito appearance after so long ;-; I was freaking missing our conceited conniving thief damnit!
Everyone is saying that it's Vermouth because Kaito isn't too dumb but I don't think that's the case. Not that I think Kaito's dumb or anything because if he was, then he wouldn't have been able to survive as a thief for so long but I think Kaito has a plan, including that smile.
Honestly I see why people think it's Vermouth. She's not that smart despite her cunningness that she can't achieve Kaito's. But I have a feeling that she would also pay attention to detail or else she wouldn't be able to go far in her disguising skills and we've seen how scary that's been, being able to fool almost everyone as Araide-sensei. I'm not throwing away the possibility that it's her though. If it was her, I could see her planning as far back as putting the flyer in Kudo residence and disguising as a waiter to food poison Yuusaku/Yukiko and as a result, lure Conan in. But there's a major flaw in this. If Akai wasn't busy for FBI, then Yukiko would have called Akai to fill in the shoes instead of Conan, seeing that he could disguise as Yuusaku with the similar build and with his voice changer, as well as his intelligent mind, he would have no problem substituting as Yuusaku. Also, even if it weren't for Akai, for what reason Vermouth would have to infiltrate Kudo residence using Kaito as a guise? She already knows Yukiko personally and she knows Conan's identity, I see no benefit for her to go through such efforts for this.
Like I said I could be completely wrong but I'm more leaning that it's Kaito Kid because he has more of a reason to do this. First off, Kaito never wants to leave his debts unpaid so he would try to repay it as soon as possible before it hits him harder. And.. maybe through this act, Kaito could confirm his suspicions about Conan as Shinichi.
Remember in the last cases we met him, we were given hints that Kaito was given a faint idea that he knows Conan's identity, especially with a freaking big hint that he wasn't called Conan when Kaito was disguised as Agasa? I think the biggest confirmation on Kaito's part was when he says he was up against two high school detectives. (the only high school detective visible was Heiji) What happens if Kaito is playing dumb and is letting Conan solve the case in order to see first hand his deduction skills? He immediately smiled when Conan told him to come in and give the overview, like he is able to gain more information from there on for his objective. Like we said, he's been acting too dumb when he was solving the cases, so inattentive to details, and he immediately resorted to, "hey you know the case so how about you explain to me?" like he wanted to see Conan's skills and deliberately explain to him and especially with so many evidence in the house to point it was Shinichi, he could easily corner Conan to reveal his identity. I mean at this point, Kaito Kid would be a very viable ally so Conan should honestly reveal his identity to him. If anything, Conan can do some sleuthing himself to blackmail Kaito into his identity as well so win win.
I'm sticking to the fact that this is Kaito himself and not Vermouth. His gestures are way too Kaito Kid-like, such as calling Conan a tsundere in that manner and hitting on girls. Heck he's even calling crime scenes disgusting! There's just so much humor in him that is too Kaito Kid like. Maybe it is Vermouth but I feel Vermouth wouldn't go through the playful gestures that is so well-known with not just Kaito Kid but Kuroba Kaito.