Hi. I don't use Vray, but I can help you with some of your questions.
- The environment map will always be outside any object in the scene. So you can't make the dome bigger and have the environment inside.
- I know this (oldschool?) technic with using a dome, but if it doesn't fulfill a very specific function that can't be reached any other way, then I wouldn't use it. If you only want to use it as a reflection map, then put the sky texture in the reflection override slot instead.
Hope it helps