You may be receiving an ‘Error 691’ or similar when trying to make a VPN connection in Windows 8 that worked just fine in Windows 7 or earlier. I fixed the issue with the following VPN connection’s configuration. Your mileage may vary depending on your VPN server configuration:
Open the VPN connection properties by right clicking on the connection in the Windows 8 network charms bar and choose ‘View Connection Properties’. Click the ‘Security’ tab. Make sure ‘Allow these protocols’ is checked (the ‘Microsoft CHAP’ check-box is already selected).
My VPN finally connected without issue. Good luck!