I have successfully used the VPN with the Padavan firmware. ( used the Asus DNS, I set it up under advanced/wan/ddns and then keep trying for the account that i want etc) While I have not tried the VPN wizard on windows 7, I have been able to connect to the VPN through both my android phone and from my Apple laptop.
I set it up as pptp, set encryption to MPEE-218, broadcast to off (all other settings kept at default), then add the login and password in the account section below ( I leave the ip area blank).. (the login and password are both cap sensitive fyi)
This setup seems to work for me, so if your settings are similar.. the connection issue may be on the windows side?
Hope this is helpful