http://www.kevssite.com/2009/04/21/using-vi-as-a-hex-editor

Sometimes I find it useful to switch to hex mode when editing a file in vi. The command for switching is not very obvious so thought I’d share… So, open a file in vi as usual, hit escape and type:

:%!xxd

to switch into hex mode And when your done hit escape again and type:

:%!xxd -r

to exit from hex mode. Okay, so this isn’t actaully switching to vi’s ‘hex mode’; vi doesn’t have one. What the above actually does is to stream vi’s buffer through the external program xxd.