Supplementing works. The fact that it's fat-soluble just means it gets stored in your fat cells. I use this:
https://store.veganessentials.com/vegan-vitamin-d3-capsules-by-global-health-trax-p3609.aspx
or this:
https://store.veganessentials.com/vegan-vitamin-d3-capsules-by-the-food-movement-p3730.aspx
depending on availability. I take 5000 I.U. every other day, but you could take 5000 I.U. a day until your numbers become normal. (You can overdose on vitamin D. When you take too much, it builds up in your body over time, to the point where it becomes toxic. So don't overdo it. Dr. Greger recommends 2000 I.U. per day. Since it gets stored in your fat cells, you could, for example, take 14,000 I.U. once a week, and it would amount to the same thing.)
According to this article:
https://examine.com/nutrition/how-much-fat-do-i-need-to-absorb-vitamind/
taking a vitamin D supplement with no fat can reduce its absorption somewhat, but so can taking it with too much fat. So I wouldn't overthink this. Just take the supplements, monitoring your blood level to see if it's going up accordingly.
Incidentally, a vegan diet is not necessarily "very low fat". In fact, you would have to work hard to make it low-fat. Note that vegetable oils (e.g., olive oil, canola oil) don't come from animals. Furthermore, even vegetables and fruits, not to mention nuts and seeds, have some fat content. So even if you avoided all cooking oils, you would still be ingesting some fat -- which is a good thing, since your body needs it.
If you absolutely want to make sure you're taking your vitamin D supplements with fat, I suppose you could eat a handful of walnuts or some avocado beforehand. Dr. Greger just recommends taking it with the largest meal of the day:
https://nutritionfacts.org/topics/vitamin-d-supplements/