It is pretty much a requirement. You'll need to use in a dome port. See the explanation here: http://ikelite.com/w...s/qdiopter.html
I read some people use a diopter with this lens. Would that be a requirement, or can I get acceptable result without too?
I have found that if you use a 6" dome, +4 diopter will preserve the magnification at closest focusing distance. i.e. you'll get the same magnification underwater behind the dome as the lens does in air. I use a +5 with my 18-55mm to give me a little more magnification - and so I can get closer and reduce the amount of water.
If you are only going to be shooting stills it doesn't really matter. If you can get the non-VR version and it is cheaper, get it. I suspect that the non-VR are hard to get. I actually have both and the VR bought a few months ago was cheaper than the non-VR version I bought a few years ago.
And how about the choice between the VR or the non-VR version?
If you plan to shoot video, the VR version gives you a little more stability in the image.