This side story takes place kind of somewhere in the middle of Dance in the Vampire Bund. It's not super necessary to read it right when it should be taking place 'cause it is after all just a side story, but when the new character introduced here is later shown in the main story you might be like "Wait, who is that?" when he's treated so familiarly and you've never seen him before.
I may or may not come back in a while to post what chapter range of the main story this one should be taking place alongside, for anyone who would like to sort of read things chronologically. I'll have to dig through the chapters a bit to figure it out 'cause honestly, I just read the main story and then both gaidens afterwards even the second time through so I lost track of that. If anyone else wants to deal with that by all means, feel free to.