There is no linear transformation between the two. Vertex coordinate is equal to surfaceRAS when the point that you have selected is actually one of the vertices on the chosen surface. However, if the point is not on the surface, it finds the nearest vertex. So, you can read your surface  ( in matlab : [vertices, faces] = read_surf('lh.pial') ) and then find the vertex that is closest to your selected point (surfaceRAS). This is the vertex that you see in the Freeview.