It seems (unless I’m missing something…) that anything you connect to the World Position Offset node in a material is evaluated in the vertex shader, but that there is no way to create your own vertex shader interpolant outputs.
Sometimes you need a value in the pixel shader, but you know that a per-pixel computation of it is overkill and possibly quite expensive. In those cases, when writing a shader by hand you might calculate the value in the (typically much lower computational frequency) vertex shader and pass an interpolated version down to the pixel shader. However, I can’t find a way to accomplish this in the materials system, which leaves me doing more computation in the pixel shader than I really want to.