Winter is coming people!
Vitamin C has been shown to improve immune function by encouraging the body to boost white blood cell production. So battle the cold weather by incorporating some of these foods above into your diet.
References
https://www.ncbi.nlm.nih.gov/pubmed/25157026
It gets pretty overcast and cloudy in my part of the world for the winter, and I always get a bit lethargic / almost depressed from lack of sunshine, I feel.
What are some good recommendations for Vitamin D?
I feel your pain, I live in Denmark right now and the long winter nights can take their toll on your Vitamin levels. I always supplement year-round with Vitamin D3 ( the most absorbable form) 2,000IU a day.
Food-wise I favour egg yolks and fatty fish, so Omelettes and Salmon are my go-to dishes.
North latitude solidarity! I don't live that far currently, but I've been there.
Any recommendations for vegetarians, too?
Yeah, Soy milk and soy-derived products can provide a decent amount. I also like to incorporate mushrooms into my diet, that have a range of minerals including vitamin D.