View direction -vd

I was looking for some explanation concerning the -vd option in rvu and found this topic help with view direction vector
apparently the two answers would sound different (direction in the first case, difference between the point of interest and the position of your camera in the second). I have tried to apply both but was not able to get complete control on the thing. I know that this might appear a trivial question.
How would you define view direction?
I assume that the coordinates of my view point can be taken directly from a modeling software.

Thank you for your kind help!

The view direction is a vector pointing at the same way the camera is facing. This tutorial has a diagram as well as a script to get the correct values from the Blender camera object. Hopefully it helps: