Throughout the year, make sure that the skin (without sunscreen) is exposed to the sun for 15-30 minutes daily for sufficient vitamin D production.
If you are living in Europe or North America, you can get enough vitamin D from sunshine in the spring and summer if you follow this guideline, but you probably don’t get enough sun in the autumn and winter months.
Therefore, the vitamin D supplement advice for vegans is to take a daily supplement of 25 to 50 mcg/1000 to 2000 IU of lichen-based vitamin D3 in case of non-daily sun exposure in summer and spring, and in any case in the autumn and winter months.
It is wise to take vitamin D during or after a meal, because it is better absorbed taken together with some fat.
Everyone above the age of 70 should take a supplement of 20 micrograms per day, independent of their diet.