ORA-12638: Credential retrieval failed

Honestly this is one of those errors that can be caused by lots of different root problems.

On the assumption your database server is Windows, check its event log, both System and Application. You may find something there that is useful.

I have encountered this error in some cases because the account under which the Oracle service was running got locked.

I have also encountered it in cases where we never found the explanation, but we were always able to clear up the issue by having the user lock and unlock their PC.

Is the VB application running on a different machine than the client that can connect? If so, is it possible to test using a different client program on the same machine -- or is it possible to try the VB application on your machine?

I know this is a very old post but I have found a solution that has worked for me (Legacy VB6 Application):

Change the following entry in the sqlnet.ora file:

Original Entry - SQLNET.AUTHENTICATION_SERVICES= (NTS)

Modified Entry - SQLNET.AUTHENTICATION_SERVICES= (NONE)

Here is a LINK to the solution