Vitamin D is a very broad issue, too broad to cover it here. Most of the time however I spend battling the meme-bots.
Vitamin D is an issue for everybody, especially for people who live typical Western lifestyles with most of the day spent indoors. It's not a vegan issue. It's a vegan + everybodyelse-issue.
The new scream is, that we need much more Vitamin D than previously thought, namely up to 1,000IU per 25lb of body weight.
The question is not how much exactly you need to take, but what amount will get your blood levels up to the acceptable range.
If you're lucky enough to live in the US or Canada, you have access to supplemental D2 which may be the vegan type. May, because just because the Vitamin is vegan, doesn't mean the product is depending on what else is in the supplement.
Now I don't like to mention Vitamin-D-internet-busybody Dr. John Cannell, because I think he is an antivegan and he is one of the prime sources of the "D3 is superior" memebot. And while his doctorate stands for psychiatry, he does have a lot of good info on Vitamin D - if you're one of the people who can filter out the D3-is-IT-crap. His advise on Vitamin D supplementation however I like: Take 5000IU every day for three months and then have your levels checked. The thing you need to have checked, and only this makes sense, is the 25(OH)D. It should be between 50–80 ng/mL (or 125–200 nM/L) depending the unit value the lab uses.
If your 25(OH)D level is below 50 ng/mL or 125nM/L, you need to increase your dose until you reach.
Yes, European vegans don't currently have that option unless they get prescription from a doctor for a high dose medication. Thanks to the D2-is-inferior-meme, D2 as a supplement got pulled off the shelves throughout EU-Europe, anyone reading here outside the EU and within Europe who can check if D2 (ergocalciferol) is still available as a supplement, let me know.
Given the new amounts of Vitamin D required and the outdated RDA, food has become a meaningless source of Vitamin D. One exception: Mushrooms, that have been dried in the sun shortly after harvest. Mushrooms that have not been exposed to either sunshine or UVB light, contain irrelevant amounts of Vitamin D. In mushrooms as with human skin, only exposure to UVB light will generate Vitamin D. However, you will not know how much Vitamin D is created in the mushrooms, so eating them is a gamble.
Tanning beds are an option, if the lamps they use have UVB. Sometimes they use only UVA, sometimes they use a mix, but never UVB alone, unfortunately. Because even with the mixed lamps, you'll be exposed to UVA and that unnecessarily buts a burdon to your skin.
Yes, the sun is a great source of UVB, it takes between 5-45 minutes midday summer sun, full body exposure to produce around 10,000IU Vitamin D. That process is self-regulatory, staying longer in the sun will not make more Vitamin D. The paler you are, the quicker your download will complete. The color of your eyes is irrelevant to this process. Pale + brown eyes is in the 5 minute range, black with green eyes, more to the 45 minutes range.
You do not make any Vitamin D in your skin when your shadow is longer than you, be it July evening or October midday. (Readers of OZ, it's reverse for you and given the different ankle UVB radiation may be more intense.
I do indeed grow my own mushrooms and either dry them in the sun or radiate them with an UVB bulb, but it is some work to get into the shroom growing and I guess only someone who can enjoy that as an hobby will uphold the effort mediumterm... It's also a great way to recycle your kitchen waste, because some shrooms are very tolerant about what substrate you can give them and will happily accept potato peels and coffee-grounds, once sterilized of course in a pressure cooker and according to the procedure of shrooming.
If you are time constrained, try to spend the time in the sun with as much skin exposed, when your shadow is shorter than yourself as long as required for your skintype to reach maximum production of Vitamin D.
In some cases, this will still fail. I mean who of us can actually spend between 5 and 45 minutes in the sun, almost naked, every day from March to October? I can't, and frankly I have better things to do.
For those who are already deficient, say 2 billion people, a high dose treatment may be the only route with normally 50,000 IU given once a week, I've also been told of a medication called Sterogyl which has doses of 20,000 IU, so that could be taken every second day. However much is needed to get your levels up.
I should probably cut this down since it's so long already.