DCTP Forums

I believe it was originally on fanfiction.net but I think it disappeared, or perhaps I'm just going crazy.  I think the beginning starts with a Kid heist, then Kid and Conan at some point are about to collaborate on the Black Organization but then Conan gets kidnapped by Vermouth, then towards the end there is some big reveal about Akako.

That's probably 'When Pandora's Box is Opened' by mangaluva and it is indeed on FF.Net

Agreed.
And it's still there on fanfiction.net.
Have fun ... it's an amazing fanfic.