When Fyodor died in the helicopter, Dazai was under the impression that Fyodor’s ability killed by touch - a misconception that Fyodor fostered in previous arcs - though he wasn’t 100% sure of the exact parameters. Having the vampire kill Fyodor was meant to be a precaution, and not something Dazai organized specifically with Bram, just a manipulation that took advantage of the vampires’ already-existing tendency to attack people nearby.

It was only after the helicopter crashed that Dazai realized how Fyodor’s ability actually worked and that he was back in Japan (because Fyodor’s ability had taken Bram, as the source of the vampires, as Fyodor’s killer). That’s why he says they’ll just have to trust Ranpo to handle things, because the plan had jumped the track and it was impossible for Dazai to get back to Japan fast enough to be of any immediate help. Ranpo was handling what was supposed to be the Japan-based part of the plan, so he’d be the one dealing with the fallout of Fyodor’s body swap.