The Bank Cypher is located in a small room inside the Bank of the Prophet

Go through door with green light over it:

Bankoftheprophetdoor.jpg

Then go past a room with safe deposit boxes, and turn LEFT. Find the room with "HOARDER" written on the wall:

Bioshockinfinitehoardervoxcode.jpg

This is where the Bank Vox Code is.

Full Walkthrough: Bank Vox Code Walkthrough