Hi Zoran,
Following your vServer log - "Database 'master.vdb' has been successfully closed" is ok.
Look -
(UID 1292) Database 'master.vdb' put_Location.
...
(UID 1292) Database 'master.vdb' remoteID -94947216 registered.
...
(UID 1292) Database 'master.vdb' has been successfully opened
...
The same for other users...
In other words, you intentionally do open for "master" db in your code. Usually it is not necessary.
If you want to get some info from system db/tables you can do it this way:
someDb.SqlSelect( "
USE master;
SHOW USERS;" );
// or
SqlConnection.SqlSelect( "
USE master;
SHOW USERS;" );
or even simply
SHOW USERS;