I have done some research on this issue in the past as I would love to use LogonStudioVista with x64 WS2008 but have failed so far.
I think what you may be running up a gainst is WIndows Resource Protection (http://msdn.microsoft.com/en-us/library/aa382503.aspx), which in short prevents any changes to a protected list of installed programs, which this dll is one of. I have experienced replacing the file (even under safe boot), and finding the system quickly replaces it back! You need to use a particular set of APIs, that for example Windows Installer uses, to make a file change stick.
I concluded to myself this was the issue, there in fact may be even more robust protection under Windows Server 2008 x64 but I can’t remember now as its been 6 months since I researched this.
Good luck if this helps you make some progress!