Well, then, explain that part of The Rules.
The essential point is that once a vacancy is announced, the process for replacing the Leader begins pretty much automatically; the NEC has to meet to decide a timetable, after which nominations have to be completed, while a fairly long period between nominations and the Special Conference is needed because fo the way the voting works. Which means that any party that wants to deal with Labour but not Labour led by Brown would have to wait for quite a while for a new Leader to be elected with no way of being sure that the new Leader is someone they'd like to work with.
I'm pretty sure that a coalition might also have to be approved through formal procedures, though I'm not entirely sure about that.