Vitamin D helped me too, it seemed to increase my energy.
It makes sense to add this to your diet if you're working behind a desk indoors most of the day.
It's my understanding that "overdosing" on Vitamin D is very difficult. I take 5000 IU every morning. Do you have links to the research you found?
From Steve Gibson's Vitamin D 'fan page' http://www.grc.com/health/vitamin-d.htm:
John Jacob Cannell, M.D. and executive director of the Vitamin D Council has this to say about Vitamin D Toxicity: http://www.vitamindcouncil.org/vitaminDToxicity.shtml
The Merck Online Medical Manual ( http://www.merck.com/mmpe/sec01/ch004/ch004k.html ) has the following to say: “Because synthesis of 1,25(OH)2D (the most active metabolite of vitamin D) is tightly regulated, vitamin D toxicity usually occurs only if excessive doses (prescription or megavitamin) are taken. Vitamin D 1000ug (40,000 IU)/day produces toxicity within 1 to 4 months in infants. In adults, taking 1250 ug (50,000 IU)/day for several months can produce toxicity.”
Please note that my depression was minor, more of the "there is too much going on, I can't get started on anything, I don't feel like working, I think I will just browse something new on the net" day after day, sometimes for weeks at a time. Also, I am convinced everyone's system is different, so tread carefully and consult professional medical advice, where appropriate.
I also had recently (4 years ago) moved to way northern California, US, along the coast, so I get Oregon's type of weather (much more rain, overcast). Sun was no longer so prevalent, and very much may have had a bearing on this. YMMV.