A file secret.txt to be encrypted better be edited without leaving traces such as its path, swap files, backup or undo data. To achieve this in Vim from the shell (say Bash or ZSH), open it by the command vims secret.txt using the shell alias

vims=" vim -n -i NONE \
  +'set backupskip=*' \
  +'augroup Secure | autocmd BufWrite * setlocal noundofile | augroup END' \
  +'silent doautoall Secure BufWrite"
alias vims="$vims"
unset vims

added to the configuration file (~/.bashrc respectively ~/.zshrc).

If the secret file is a temporary file, for example, kept in /tmp on Linux, then the same can be achieved automatically, without any alias, using the following permanent Vim settings

"  don't store marks of temporary files in temp or shm directories
let &viminfo .=
      \ ',r' . (has('win32') ? $TMP : $TMPDIR) .
      \  (exists('$XDG_CACHE_HOME') ? ',r' . $XDG_CACHE_HOME : '') .
      \  (exists('$XDG_RUNTIME_DIR') ? ',r' . $XDG_RUNTIME_DIR : '') .
      \ ',r/var/tmp,r/dev/shm,r/run/shm,r/var/run/shm' .
" don't backup temporary files (such as those in temp or shm directories)
let &backupskip .= ',' .
  \ '/var/tmp/*,/dev/shm/*,/run/shm/*,/var/run/shm/*,' .
  \  (exists('$XDG_CACHE_HOME') ? escape(expand($XDG_CACHE_HOME), '\') . '/*,' : '') .
  \  (exists('$XDG_RUNTIME_DIR') ? escape(expand($XDG_RUNTIME_DIR), '\') . '/*,' : '') .
" don't keep swap file or undo data of temporary files
augroup vimrc_skip
  exe 'autocmd BufNewFile,BufRead' &backupskip 'setlocal noswapfile'
  exe 'autocmd BufWrite' &backupskip ' setlocal noundofile'
augroup END
silent doautoall vimrc_skip BufNewFile,BufRead,BufWrite

added to the configuration file (~/.vimrc in Linux and MacOS or %USERPROFILE%/_vimrc in Microsoft Windows).

Otherwise, open the file in Vim by the command :EditSecurely secret.txt given by

command! -nargs=1 -complete=file_in_path EditSecurely set viminfofile=NONE | edit <args> | setlocal noswapfile nobackup noundofile

Once a file has been encrypted, say by Gnu Privacy Guard, conveniently edit it using the vim-gnupg plug-in without leaving traces.