Just so we're clear, xxd
is not a vim command; it is an external program that translates to/from hex dumps. The command %!xxd
means 'run the external program xxd
, passing it the contents of this file via stdin
, and replace the contents of the file with the result.'
From that, I hope you understand that you are not using some special mode of vim to edit these hex dumps. The hex dump is simply the text you see, and you are editing it as a normal text file.
There may be some extension to vim which provides the functionality you are looking for (I haven't looked very hard), but in answer to your question, there is no built-in functionality to do this.