How do I open a .dat file in Windows Vista?

A .dat file should be noting more then a flat ASCII file. Did you try to open it in "Notepad?"

You can try here: MS-TNEF degenerator (http://tud.at/php/tnef/)

Depends on what created it. Dat files have no native format. Sometimes they are plain ASCII, other times they are a format native to the application that created it.