#!/bin/bash

BACKFILE=${1}.bkp

if [ -e $1 ]; then

if [ -e $BACKFILE ]; then
        echo "Overwrite \${BACKFILE}? y/n"
        read
        if [ $REPLY = y ]; then
                bak()
        else
                break
        fi
else
        bak()
fi

fi

vim $1

function bak { mv $1 $BACKFILE; }