Tools released at Defcon crack MS-CHAPv2
I hope your network is using a more modern and more secure authentication protocol than MS-CHAPv2 for VPN access, but if it's one of those that's still running PPTP VPNs, this is something you need to be aware of: yesterday at Defcon a crypto researcher released tools for cracking the protocol that were designed for penetration testing - but we all know these tools always eventually get into the wrong hands.
The good news is that using the service being offered to do this is somewhat expensive ($200). The bad news is that if these folks can do it, so can other talented hackers. Maybe it's (past) time to dump PPTP and MS-CHAPv2 and look for more secure VPN alternatives for your Windows network, such as L2TP/IPsec or SSTP.