How to get vim to show a byte-by-byte representation of file data
I don't want vim to ever interpret my data in any encoding specific way. In other words, when I'm in vim, I want the character that my cursor is on to correspond to the actual byte, not a utf* (etc.) representation of that byte.
I need to use vim to analyze issues caused by Unicode conversion errors made by other people (using other software) so it's important that I see what is actually there.
For example, in Cygwin's vim, I have been able to see UTF-8 BOMs as
 [START OF FILE DATA]
This is perfect. I recognize this as a UTF-8 BOM and if I want to know what the hex for each character is, I can put the cursor on the characters and use 'ga'.
I recently got a proper Linux machine (Fedora). In /etc/vimrc, this line exists
set fileencodings=ucs-bom,utf-8,latin1
When I look at a UTF-8 BOM on this machine, the BOM is completely hidden.
When I add the following line to ~/.vimrc
set fileencodings=latin1
I see

The first 3 characters are the BOM (when ga is used against them). I don't know what the last 3 characters are.
At one point, I even saw the UTF-8 BOM represented as "feff" - the UTF-16 BOM.
Anyway, you see my problem. I need to see exactly what is in my file without vim interpreting the bytes for me. I know I could use xxd, od, etc but vim has always been very convenient as an analysis tool. Plus I want to be able to edit the files and save them without any conversion problems.
Thanks for your help.
Answer
Use 'binary' mode:
:edit ++bin file
or
vim -b file
From :help 'binary':
The 'fileencoding' and 'fileencodings' options will not be used, the file is read without conversion.