Nikon 18-55mm kitlens

I read some people use a diopter with this lens. Would that be a requirement, or can I get acceptable result without too?

It is pretty much a requirement. You'll need to use in a dome port. See the explanation here: http://ikelite.com/web_pages/qdiopter.html

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.

And how about the choice between the VR or the non-VR version?

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.

If you plan to shoot video, the VR version gives you a little more stability in the image.

I tried the old version 18-55 EDII in a night dive with the combination of a 6" dome, 13mm + 20mm extension, +4 diopter in a Nauticam housing. This let me took pictures of creatures from big frog fish, subjects from big and small like small octopus, flounder, leave fish...

The above discussions are very informative. The corners improved a lot comparing to just using a flat port. The original wide view can also be restored.

I love this setup because I can take a wide range of subjects underwater and topside. In addition, i can use the tokina 10-17 zoom gear by adding a plastic hood with velcro tape.

I know this is kind of out of the context, but is the canon 18-55 is as the same level of the nikon ones?