My university uses PPTP for its VPN, luckily GNOME’s NetworkManager has a PPTP plugin available for it called ‘NetworkManager-pptp’ but despite entering the correct settings, username and password it used to try to connect and always fail.
After much searching I found this forum thread with reports from many others also experiencing this problem, this post in particular solves the issue, so I didn’t see any need to reproduce its clear instruction.
It involves using ‘gconf-editor’ to add some entries, a bit like editing the Windows registry.
Hope it helps someone.