Vit D only causes harm in VERY high doses over a LONG time, as far as I am aware. My partner is vegan - any danger relates to potential build up of calcium.
10mg a day if you are not deficient is fine. If you are deficient, then you can increase it over the winter months but drop back during summer.
If I can find any links to support this, I'll post them here. My partner is on the advisory board of the Estonian Vegan Society so has access to a wealth of help.....Vit D is a big deal here......