Usernames sent by Windows are case-insensitive; however, usernames stored in the UNIX user database are case-sensitive. The authentication server converts all Windows usernames to lowercase before working with them.
This means you must set up username mappings for all users with mixed-case UNIX usernames. If the authentication server can't work out the user's UNIX username -- for example, if there's no username mapping -- then the user is granted guest access.
Otherwise, passwords are checked against users' Windows usernames.
The VisionFS server uses the standard UNIX mechanisms for accessing the UNIX user database. For example, it doesn't matter if the UNIX server uses /etc/passwd or NIS -- VisionFS checks whatever you're using.