How do I make file associations in Vista?

In Windows XP I would open Windows Explorer and do TOOLS -> FOLDER OPTIONS -> FILE TYPES.

Where is the equivalent in Windows Vista?

I think it was removed from the Explorer user interface in Vista. You can still access this functionality through Opus.

and where in Opus do I go?

Settings / Filetypes

The Opus File Types editor that Tanis pointed you towards is more powerful but Vista does have its own association editor under Control Panel -> Default Programs -> Associate a file type or protocol with a program.

Vista also has the Set Default Programs option in the same place, which allows you to tell a program to re-claim all of its filetypes, if that program supports the new Vista API for association editing.