vimdiff functionality

[Lev Lvovsky]

I'm using a version control app called darcs, which allows me to view  
a diff hunk by hunk, to pick and choose the changes that I want to  
apply.  darcs aside, is there an in-depth howto for editing patches  
and such?  Specifically, I'm looking for a way to split single hunks  
into two hunks, write the diff, and then feed it into darcs.

My co-worker uses emacs, which seems to come with a pretty complete  
patch editing command set - it's likely my inexperience that's  
causing me to not find much of that same functionality in vim, so any  
pointers would be appreciated...

thanks,
-lev