To avoid more smartass comments: This IS IN D MAJOR. They are playing with A tuned to 415Hz as oposed to today's more common 442Hz. It's the tuning that is a half step down. Not the key.
For high quality:    • Vivaldi - Gloria in D Major, RV 589  
Antonio Vivaldi - Gloria in D Major for 3 solo voices, chorus, trumpet, oboe, violin (Ad lib), 2 violas, 2 cellos, strings and basso continuo, RV 589

1 - Gloria in Excelsis Deo

2 - Et in Terra Pax

3 - Laudamus Te

Performed by: The English Baroque Soloists and Monteverdi Choir
Directed by: John Eliot Gardiner