Vitex needs full sun. But I suspect your problem is you're just in too cold an area. It doesn't bloom for me either. I'm in zone 6. I think they're reliably hardy to about zone 7. I know you said it's in a protected area, but that still might not take care of it. We get beautiful foliage here, just no flowers. Same with a lot of other marginally hardy shrubs.
Have you seen vitex blooming anywhere else in your area? I know you're about the same climate as we are.
