OK - this is how you can fix the problem.
make a backup copy of your network id pref list, perhaps using the command line as below from a terminal window.
cp /Library/Preferences/SystemConfiguration/com.apple.network.idenfication.plist /Library/Preferences/SystemConfiguration/com.apple.network.idenfication.plist.o ld
open Library/Preferences/SystemConfiguration/com.apple.network.idenfication.plist using a TEXT editor or something similar.
Open the network settings screen where your VPN is listed (under System Preferences) and make a note of your VPN settings. Then delete the VPN using the "-" button and click Apply.
In the com.apple.network.idenfication.plist you will see the the defintion for the VPN settings a little like thus:
<dict>
<key>Signatures</key>
<array>
<dict>
<key>Identifier</key>
<string>VPN.RemoteAddress=myvpn.domain.com</string>
<key>Services</key>
then lots of settings info, and ending with the definition of a new service
</array>
<key>Signature</key>
<string>VPN.RemoteAddress=myvpn.domain.com</string>
<key>Timestamp</key>
<date>2009-09-01T06:42:53Z</date>
</dict>
<dict>
<key>Identifier</key>
Delete everything and including between the <dict> and </dict> for this Identifier and this will remove the VPN settings and leave all the others intact.
Save the file.
Now go back to your network settings screen and re add the you VPN and it will work once again as it did before.