Why, yes, it is! I've seen people speed off of an edge to a lower ground.
I have seen better examples, but this is the only video that I had in my favourites. The others I don't remember the names of. Here is a speed off of a ledge:http://www.youtube.com/watch?v=15cf7YVVKesAt 1:56. I encourage you to watch the entire video, as it is very good.
Great video, But I still wouldn't call that a vault
Here is a pretty low speed vault:http://www.youtube.com/watch?v=eFmxQBShtrQ2:44