1. A peaceful solution would be to consider sqrt[Dx^2+Dy^2+Dz^2] as "amplitude".

2. If I were you, I would give Vendor a table with f, Dx,Dy,Dz, and sqrt[Dx^2+Dy^2+Dz^2] for the points considered, maybe Vendor wants to perform a generalized Fourier model; anyway it seems that Vendor knows better what he wants. Why stop at 33 Hz (this is a limit coming up from other topic), give Vendor more and see what Vendor says...
I'm new too in this kind of topics, where the Valve Actuator is like the hammer of Thor. May the mystery of Vendor Actuator be with us!