You can visual with your eyes open but it's more effective if you can close your eyes. That's just because the images can be more vivid and you can be focused specifically on your visualization. If you can apare 10 minutes a day I would suggest that. I usually do it throughout the day in between breaks at work and while I'm driving. Daydreaming is visualization so I use my day dreams to visualize what I want.

You can visualize with your eyes open for sure. For the most part, I think it's the feeling(s) that comes along with your visualization that really sets things into motion and helps to bring your desire to you. I've read that numerous times, that what you feel is most important. So as long as you're still feeling positive feelings, or feelings that you would experience if you already had your desire, it shouldn't matter if your eyes are opened or closed.