hi, I'm new here and new to modding....this is my first attempt.

I have created part of vault 74a and was going to go inside and check it for placement errors. I loaded the FO3 launcher and went to the data file, checked the vault74.esp box and clicked ok...re-opened the data file and the check mark was gone! I went through this several times and it keeps removing the check mark. How do I fix this?

never mind! I hadn't installed my DLC yet. Problem solved

The Fallout launcher's mod manager is bugged; it always deselects all the modules when it's opened. I don't know if it was ever patched. Most people use a 3rd-party mod manager instead, the one I use is called http://www.fallout3nexus.com/downloads/file.php?id=640.