zsh/Functions/Zle/edit-command-line

# Edit the command line using your usual editor.
# Binding this to 'v' in the vi command mode map,
#   autoload -Uz edit-command-line
#   zle -N edit-command-line
#   bindkey -M vicmd v edit-command-line
# will give ksh-like behaviour for that key,
# except that it will handle multi-line buffers properly.

local tmpfile=${TMPPREFIX:-/tmp/zsh}ecl$$

print -R - "$PREBUFFER$BUFFER" >$tmpfile
exec </dev/tty
${=${VISUAL:-${EDITOR:-vi}}} $tmpfile
print -Rz - "$(<$tmpfile)" 

command rm -f $tmpfile
zle send-break		# Force reload from the buffer stack