I finally had a bit of success at least in the matter in getting this vector displacement to sit level with the ground *.
I should say that this exr displacement map I'm using is nothing fancy, just a crude one I made in the sculpting features of C4D using the RGB (xyz object) option.
Essentially in Terragen I have subtracted 0.25 from the X,Y,Z vectors. But it did not work until I first broke them separately, while keeping them as vectors.
I found the amount 0.25 by trial and error.
I piped the Image Shader to a Multiply Vector 0,1,0 to eliminate the green and blue and isolate the Y green part of the vector.
Then used a Subtract Vector of 0,0.25,0.
I did a similar process on each of the X and Z vectors isolating each one separately and then subtracting 0.25.
The three portions needed to be sent to the inputs of a Build Vector and then on to the Vector Displacement Shader.
Kind of messy but, haven't been able to further combine or simplify without spoiling the displacement.
( * overall planet surface altitude is still modified by this method)