Looking at you query string I see you are specifying username and password, and that means you are providing SQL Authentication credentials. However in your question you state SQL Authentication is disabled in the SQL server.
That's the reason why your SQL server is not letting you in. You are providing SQL authentication credentials when your server is not expecting them.
If SQL Authentication is disabled, then is very likely your server is not in Mixed Mode Authentication either so that pretty much means the SQL server is using Windows Authentication for logins. This configuration is beyond your control, it's your DBA who made that choice.
Therefore, if the SQL server you are connecting to is using Windows Authentication mode, your query string should be:
cn.Open "DRIVER={SQL Server};Server=123.456.345.567\instance;Database=MyDB;Trusted_Connection=yes;"
However here comes the tricky part... if you are running your VBScript from your test machine that is part of [DOMAIN_A], it will run with under a [DOMAIN_A] windows credentials. However your SQL server is expecting windows credentials from the domain he is joined into. Let's call it [Domain_B].
Unless you AD admins have created trust relationship between [DOMAIN_A] and [DOMAIN_B] your login attempts will still fail, despite now having a correct query string.
How do you solve this cross-domain issue if you can´t join your test machine to [DOMAIN_B]? Easy. Use the "Runas" command.
On your test machine joined to [DOMAIN_A], open a command prompt and execute:
Runas /noprofile /netonly user:[DOMAIN_B]\myuserid %comspec%
It will ask you for the password. Enter the "mypass" password you originally had in your query string. "myuserid" is also the username in your original query string.
After doing that, you will have a new command prompt window, but if you notice in its title, it is now running under your [DOMAIN_B] credentials.
Despite still being on your test machine joined to [DOMAIN_A], anything that you execute on this new command prompt will do it under your [DOMAIN_B] credentials. Exactly what your SQL server is expecting.
So at this point, only thing left is to run your VBScript on this new command prompt window. That should provide the appropriate credentials to SQL server.
Let us know if that solved the issue.