TopologyVertices is a bit tricky, the numbering is not the same as the one used in the mesh. There is everything in RhinoCommon to cope with this. There are some ressources on this forum. 

http://www.grasshopper3d.com/forum/topics/mesh-topology-vertices

http://www.grasshopper3d.com/forum/topics/using-the-topology-functi...

Deconstruct mesh to vertices, deconstruct vertices (points) to X,Y,Z values. Sort Z values and then sort points at the same time. Use list item to find the first points on the list. Reverse list to find the last points on the list.

Just noticed the reverse is on the list item, not the point list. But you get the idea.