Is it possible to get back data for a specific vertex from a shader ?

I have come across this problem twice, in two major systems i am working on.

I would like to find the color or height in world space of a specific vertex that a shader is applied to.

Is it possible to have one public shader value (vector3) and when i am in the vertex of interest (also an issue of how to identify that vertex) assign a property directly from the vertex shader code to that public value and read it in script ?

Thanks in advance for any help