How do I convert a frile from Vista to Windows 2003?

The file or document format is not associated with the operating system, but with the program that created it. Files created in Vista should work fine with the same program (or a newer version of it) in Windows 2003.

Are you referring to a document created in office 2007 to be read in office 2003 or older.
In which case you need this: Download details: Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats

There is no such thing as a Vista File or a Win 2003 file. File formats are dictated by the application that created them. Windows is not an application but an operating system under which applications can be run.

So the question is what application created the file and what versions of the application do you want to use the file in?