You can cheat using the filteExpand command instead of ls. FilterExpand without arguments always works on the current selection, and the selectionMask option limits it to a particular component type. So
cmds.filterExpand(sm=31)
will return only the selected vertices and nothing else. If there aren't any verts selected it will return None.
The other way is just to check the selection for the string '.vtx' which is what you'd get from selected verts
selected_verts = [v for v in cmds.ls(sl=True, fl=True) if '.vtx' in v]