The following code fails after Windows Vista SP1 is applied. This has been tested under Windows Vista Business 64-bit with Samba version 3.0.23a and Samba version 3.0.28a.
BOOL bRet = LogonUser(TEXT("username"), TEXT("DOMAIN"), pwd, LOGON32_LOGON_NETWORK, LOGON32_PROVIDER_DEFAULT, &hToken);
GetLastError() returns 1326, ERROR_LOGON_FAILURE.
If LOGON32_LOGON_INTERACTIVE is used, LogonUser succeeds. It also works fine without SP1.
I have not tested with the computer joined to a regular Windows domain.
Can you please provide a network trace of the failing action (and samba logs as well) ?