[Shape keys][0] only store the vertex positions, so a matching number is all that's required.

If you run `bpy.app.debug = True` in the [python console][1], you'll get a Indices option in the [Mesh Display panel][2] of the Properties shelf. With it enabled you can see the index numbers of vertices — these are what's used to find the corresponding point to move to.

[0]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Animation/Techs/Shape/Shape_Keys
[1]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Extensions/Python/Console
[2]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Modeling/Meshes#Mesh_Display