Signs of Diabetes

Could a lack of vasopressin be the cause of insomnia and excessive thirst?

There is no evidence of diabetes after many tests and there is not excessive urination at night. The sleep disturbances have not been helped with "sleep hygiene" methods. There is very vivid dreaming and recall. The thirst is high-moderate at all times with bouts of sudden great thrist, which if not eleviated quickly cause dizziness. There is also a history of brain trauma.
There isn’t excessive urination compared to the amount of water intake and urination is not the primary cause of insomnia. While there have been possible indicators of pituitary issue Prolactin levels seem to be in normal range. (Hx of thyrioditis-resolved.) This is curiousity based not looking for a diagnosis.
Any dosage of Melatonin triggers headaches. Water consumption is high so urine out put is probably comparable. It is not unsual to drink a liter of water while "sleeping". Psychotherapy and sleep hygiene have not relieved symptoms. Symptoms have been on-going for 20 yrs.

Vasopressin is an anti-diuretic hormone, have a look at this site for more information:

http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/hypopit/adh.html

A lack of vasopressin should cause polyuria (excessive urination) which is required to avoid brain damage during to the excess of water ingested because of the bursts of thirsts. You say you have an history of brain damage, which might suggest that this regulation is missing in your body.
While there is evidence that a lack of vasopressin can cause excessive nocturnal urination, nothing suggests that this hormone can cause similar symptoms "per-se". I would rather think that your insomnia is of a nervous origin. Try seeking for help with psycotherapy or hipnosis. Yoga and herbal remedies could also help a bit.

I suffer from insomnia too and, although if I lay awake in my bed I tend to visit the bathroom often, I don´t really "need" to empy the bladder, it is just an anxiety-related response.

Good luck