If you're looking for classical duets with a male and female singer with less vibrato, here are a few suggestions:

1. "The Flower Duet" from Lakmé by Léo Delibes

2. "Au Fond Du Temple Saint" from The Pearl Fishers by Georges Bizet

3. "La ci darem la mano" from Don Giovanni by Wolfgang Amadeus Mozart

4. "Bei Männern, welche Liebe fühlen" from The Magic Flute by Wolfgang Amadeus Mozart

5. "Là ci darem la mano" from Don Giovanni by Wolfgang Amadeus Mozart

These duets feature beautiful melodies and harmonies with smooth vocal lines that could be perfect for a relaxing listen before bed.