Vasopressin is an anti-diuretic hormone, have a look at this site for more information:
http://www.vivo.colostate.edu/hbooks/pa ... t/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.