ok, it could be any number of things. First, it could be a thyroid condition--mild hypothyroidism. this is not to say that it's a major condition that would require any number of medical treatments. It could be so mild as to not even show up in the ranges of different tests. And, the 'fixes' can also be diverse and natural.
To determine this, you can do any number of things. First, you can get blood work done with a full spectrum thyroid scan. You'll also want to check for nutrient deficiencies and check your cholesterol and homocystine levels. All of this may come back normal.
Next, you'll want to take your basal body temperature. For women, fertility charting is a good way to determine overall gynocological and thyroidal health. For information about fertility charting, i recommend the book Garden of Fertility
by Katie Singer. This not only outlines how to chart, but how to use basal body temperature to determine if there is a thyroid malfunction--even one so slight that it comes up 'normal' in the blood test ranges that doctors use, or one that is special to you, that is, it looks normal according to the ranges, but for your specific bodily functioning, is running too low (or possibly too high). I've also learned through some extended research that if men take a basal body temperature and notice that it runs regularly lower than 97.5 degrees F, that it's likely they also have a small thyroidal problem.
The easiest fix for this is to do things that stimulate the thyroid naturally, such as taking a full-body cold rinse after a hot shower or other heat-producing activity. This includes getting your head completely wet in the cool water. This often stimulates the thyroid, and you may notice a drastic reduction in many symptoms of mild hypothyroidism.
Soy products can cause thyroid problems in both directions. If you are a vegan who relies heavily on soy products (more than 35 grams a day), then this could be the cause of the problem for you--assuming that it is a thyroid problem. A vegan can easily avoid soy products and still recieve adequate nutrition, as many vegans are soy sensitive and cannot consume soy due to allergies but are also able to maintain veganism nonetheless. Joanne Stepaniak has a cookbook for such vegans, at least she was working on one the last i knew of it.
As for other vitamin deficiencies, if you can't get enough from the sources that you are getting, or you can't bring yourself to consume more of what you already consume AND you're not finding vitamin supplementation to be helpful (which does happen), you may have to consider forgoing veganism. I know (from my own experience) that this is a very difficult pathway for a variety of reasons. But, experimentation is really important.
Your best bet is to research as much as you can about micronutrients that may impact hair health and such. Vitamin D is one such nutrient--as you mentioned--and there may be others. Vitamin D is most bioavailable in sunlight and in raw (unpasturized) dairy products (milk products). If that is your problem, you can experiment with added nutrients (a separate vitamin D supplement) or with a fortified juice (as some orange juices are supplemented with vit D) or you can decide to add in raw dairy products.
But, as i did, i would exhaust all of your options first, as best as you can. YOu may not have years and years to experiment to find exactly the right thing, without extreme detriment to your health, so i hope that you find out what it is and find a viable solution (hopefully vegan) soon.