I also wanna correct the previous posts I made. Thanks to Eric who pointed this out, the Transform vops (current space to NDC) are not needed (that's why it was exporting different values in X and Y). Houdini just converts everything in shaders to camera space by default. So, as NDC is normalised to the output frame ratio, it's not appropriate for exporting accurate motion vectors. Here I ' ve attached a hip with the fixed shader and a Nuke script with a direct comparison between the old shader and the new one without the space transform vops.
moVecTests.rar