Many of us have had vision issues with mirena. For me, at first I had really spacey, foggy vision - like a bad TV or something, that sometimes would get dark even when the lighting wouldn't change, sometimes things would look surreal, just some really weird things. I've had mine out for 4.5 years now, and I still have issues. It's hard to describe but it feels like things are moving when I'm still. I also get the pressure behind my eyes too. I had my vision checked a few months ago and it's 20-20, but... One person to look up is ProudMomma. I know she lost much of the vision in one of her eyes, and I think it's one of the things that hasn't improved much after removal. Hopefully she will chime in, but you can click on her name to see her back story and she has a blog as well.
I would RUN, don't walk, and get the thing out before you lose your vision! It will only get worse. Of course the dr's will deny it's related, but so many of us with the same issues can't be wrong.