It's kind of like, there are 3 ranges for provoke. One for aggro, one for the pull, and one for the slowdown.
The pull has a semi circle-like range in front of you. The aggro has a full circle range around you (maybe a little bigger than the pull range). and the slowdown has a semi circle-like range in front of you but bigger than the pull's.
As for mobs that voke has no effect on. I know that for myth mobs, ice wall doesn't work on them. Maybe voke has a similiar property, or maybe not.