As you know, the PPTP VPN protocol has essentially been deprecated by Microsoft because of security issues related to credential exchange. For that reason, many people are moving away from the PPTP protocol.
However, if you still love your PPTP and want to keep it, there is an option – you can implement PEAP-MSCHAPv2 authentication.
For details, check out:
DEBRA LITTLEJOHN SHINDER
MVP (Enterprise Security)