As I remember vectrosity requires an orthographic camera for it's line drawing, so if you have a perspective camera for your 3d then you need an ortho cam above or beneath, where the vector 2 values need to be normalized to correspond. I might be talking through my hat, so let me just look at vectrosity again. Coincidentally I was planning to do something similar soon.
EDIT: Checked it briefly and I was mildly talking through my hat; as the vector camera creates itself and normalization isn't necessary, but may be one way to get it on screen. Have you played with the vector cameras rect, position and FOV, to get your line onscreen?