I realize this is outside the box of CO, but you guys are good, so I thought I would try
results As VDF.Vault.Results.LogInResult = VDF.Vault.Library.ConnectionManager.LogIn("servername", "Vault", "username", "password",
code above works in 2016, 2017 to authenticate but with 2018 it returns a false all the time.
Autodesk says to replace a DLL, that doesn't fix it either, just wondering if in your workings you have seen this or have some insight.
Sorry for the late reply. I had the same issue when the "clmloader.dll" was missing in the output folder. It can not be referenced but it has to be in the same folder and in the correct version (x86/x64). I don't know if that is the DLL that you have already tried but it resolved the problem for me and I was able to connect to 2018.
I also found this in the Autodesk Forums on how to dynamically resolve the reference to the correct platform version.