This is part of the init script, so you need to modify it:
# Server
sed -i -r -e "/^(require|refuse)-/s/^/#/" /etc/ppp/options.pptpd
sed -i -e "/^\s*echo.*mppe\s/s/^/#/" /etc/init.d/pptpd
/etc/init.d/pptpd restart
# Client
sed -i -e "/^mppe\s/s/^/#/" /etc/ppp/options.pptp
ifup vpn