I don't think exposure to light is enough to overcome an actual deficiency. When you're exposed to light you get kind of a huge dose, until you get a tan. Then your skin won't produce much more for a while.

You're right about overdoing it. There is such a thing as vitamin D poisioning. I think it draws calcium out of your bones or something. On the other hand, not enough vitamin D is bad for your bones too.

Fortunately, there are tests for vitamin D. If you think you have a problem with it, you ought to get a test.