The "VPN Details" window should not automatically open when Tunnelblick is launched.
It should open when you try to launch Tunnelblick and it is already open. (This is a convenient, one-click way to open the window if you have Tunnelblick in the Dock.)
You didn't mention what version of Tunnelblick you are using, but I will assume it is a fairly recent version. If not, you should update it.
I think that Tunnelblick is being launched twice when you log in. Please try the following:
- Quit Tunnelblick
- Logout
- Log back in
Tunnelblick should not be launched automatically. If it is, you have something (a login item, perhaps, in the "Users and Groups" System Preference) that is causing it to be launched, and you should disable that.
Tunnelblick launches itself automatically when you log in if, and only if, it was running when you last logged out (or shut down or restarted the computer, which logs you out first). You do not need to manually add a login item, and you shouldn't do so.
If you have a Tunnelblick login item, that item launches Tunnelblick when you log in. And then Tunnelblick launches itself, too (via a "LaunchAgent"). Or perhaps the sequence is the other way around, but the result is that Tunnelblick is launched twice and the second launch causes the "VPN Details" window to appear.