Bom, acho que isso resolve:
#modelo anterior modificado
sed "s/listen/listen $var_ip/" pptp.conf | awk ´ {print $1, $2}´ > pptp.conf~
cat pptp.conf~ > pptp.conf
sed "s/localip/localip $var_ip/" pptp.conf | awk ´{print $1, $2}´ > pptp.conf~
cat pptp.conf~ > pptp.conf
[ ] ´ s