Listen to Dave Swarbrick and Martin Carthy on their duo albums, But Two Came By and Both Ears and the Tail. Also Dave backing up Peter Bellamy in The Transports.

There are so many excellent examples of fiddle accompaniment to vocals I'm almost surprised at the question. Even when fiddle plays as part of a band back-up, you can get lots of ideas by just concentrating on what the fiddle is doing--in many cases, it would provide a sufficient accompaniment by its lonesome, with only a bit more fleshing out.