|Short story: Are you sure that the VPN client you are using supports Vista x64?|
You didn't say, or I skimmed to fast to be sure, but I know some the the users I support encounter this error when they attempt to install the Cisco IPSEC VPN client.
This is because the last time I checked, the Cisco IPSEC VPN client did not, nor did they have plans to provide, support for Vista x64.
Cisco has told us to use an alternative client which required that we upgrade our concentrator. that was an SSL VPN client that was installed through a website.