If I am understanding it correctly, you just need the succesful and failed login information, but instead you are stuck with a giant audit file.
Have you tried just turning on the native SQL Server 2008 R2 Security Connection logging? Simplest way that could do the trick as explained here: https://stackoverflow.com/a/6769173/2295317
If that doesn't work for you and you are looking to code up your own logging in SQL take a look at this msdn post: http://blogs.msdn.com/b/sqlprogrammability/archive/2006/08/16/703079.aspx. It's a bit old, from 2006 but it applies to SQL Server 2008 R2 and there is a walkthrough of how to set that up.
Hope this helps.