The following vocalize is in the Dorian Mode, meaning it’s in D minor without the B flat. For more info check out the Wikipedia article on Dorian. Dorian is a popular key for Medieval and Renaissance music, so it’s a good idea to get familiar with it.
This is an exercise worth getting into your fingers before presenting it to your choir. Chances are your singers will have a little trouble hearing where to sing at first so they’ll be following you on the piano.
In addition to this being an exercise in Dorian, you can also slow down the tempo and make it a sostenuto or breath control exercise. See this previous post for another sostenuto exercise.
Try to think of some variations for this exercise. Enjoy!