For reference I use a std::map for my vertex lookups. My key is a custom class that uses integers to store (x * 100, y * 100, z * 100), and implements operator<. See the TREVertexKey class here:

http://ldview.cvs.sourceforge.net/viewvc/ldview/LDView/TRE/TREVertexKey.h?view=markup

You could do something similar with your adjusted vertices. The std::map class uses a balanced binary tree, so has a lookup performance of O( log n ). Insertion is also O( log n ), but the balancing/heap allocation means that actual time to insert is longer than to do lookups. To be honest, I wouldn't know where to start to write a good hash function for a 3D vector to store them in a hash table.