I fixed it.
The instructions say to use "work with transparency" in vray dirt if you are using edge mode - but it looks like this doesn't work if you have have also selected to exclude the objects from the vray dirt calculations.
Looks like "exclude" has some odd behaviours with forest objects, and maybe even vray proxies - so I'm not sure what it's supposed to be doing.
My vray dirt element is rendering the grass completely black, even though I have chosen to excluded it from the element, and also from the vray dirt material itself.
I would expect "ignoring" to mean to leave the item white - as I mean to composite the ao pass into my render.
Has no one ever rendered an ao pass with vray over grass and ignored it?