Terrain is always chunked in A8 and thus normally in world coordinates if I'm not mistaken.
Again, three things:
Checked hit.vertex?
Tried c.z += 1; instead?
(c_updatehull for whatever reason?)

Additionally:
Have you checked the vertex number (displayed)?
You could try to calculate the vertex number manually, I think, should the previous be the problem.
EDIT: You could try to offset the hit position by the negative terrain creation position (terrain.xyz may always be 000, not sure but take care) and grab the closest vertex after that.

You can post a short and simple test project if you like to.