Print out the values and make sure it's returning what you expect. One thing I notice is that you're using vertNum as a counter. Don't do that - you already have an increment counter, i.
Code:
string $verts = ($sel[0]+".vtx["+ $vertNum +"]");
Should be:
Code:
string $verts = ($sel[0]+".vtx["+ $i +"]");
You also have two variables named almost the same thing, verNumber and vertNum. You should choose more unique names so that you don't confuse yourself.
Imagination is more important than knowledge.