When accessing a VDB with the incorrect name (in my case, a typo), the exception suggests that it's either down to an invalid logon and/or access problems. This is mis-leading and time-consuming to track down:

09:52:35,991 WARN [SECURITY] Processing exception 'VDB "XXX" version "latest" does not exist.' for session null. Exception type org.teiid.client.security.LogonException thrown from org.teiid.services.SessionServiceImpl.getActiveVDB(SessionServiceImpl.java:228). Enable more detailed logging to see the entire stacktrace.