I've seen these threads here before, like here:
thread.cfm?threadid=30596#393657

The consensus here seems to be that it's not important -- that all your favorite singers don't have/do it. You give me a list of singers who don't have it and....

1. I bet most of them DO have it
2. I'll make an infinitely longer list of singers who DO have it.
3. My infinitely longer list will contain most people's list of top five singers ever.

I think MOST good singers do have/do it, though the best are so controlled and natural that you don't notice that it is there.

This is the biggest mystery in music. You can search the web endlessly, you can take lessons, you can ask people until you are blue in the face. Nobody can tell you how it's done, where the "vibration" comes from, or how you can attain it.

I think that's because people achieve it in different ways. Some actually do have the natural thing (wherever that is generated from), while others "fake it" by huffing -- changing the volume of air higher and lower, and still others achieve it by varying pitch on a particular note.

Good luck. And if you ever find a straight answer to your question share it with me!!!