gmx distance gives you x,y,z distance between two particles (P and N).

What you want also might be found in  gmx bundle.

I don't think there is a ready-to-use tool. You might need to write some code.

gmx traj gives you the coordinates of a group of atoms (x, y, and z for each atom separately). Process the output to get the vectors.

Michal, the output of gmx distance is x,y,z coordinates of P-N vector. you only need to normalize it to get the orientation.

If someone also want to write script for such analysis, they can use xdr library to read directly for xtc or ttr files.