Hi there, welcome to the board, though sorry you're still suffering with this after so long.
Basically your vestibular system comprises three fluid filled semicirclar canals which detect where you are in space and which direction your head is moving, and the vestubular (8th cranial) nerve which transmits this info to your brain.
If this system is not working properly, the information it sends your brain is at odds with the information your brain recieves from your eyes and muscles etc, which makes you feel dizzy.
Often a virus is thought to affect this system which leads to vestubular nerve damage, though there are many different causes of such dysfunction.
Often this damage is irreversible and you get better through the development of new nerve pathways, which take over the work of the damaged part. Exercises (often Cawthorne Cooksey exercises)help to speed up this process (known as compensation).
Compensation has obviously occurred to a large degree in your case though not completely, which is why when your vision is compromised in any way (darkness/up high etc)your symptoms return.
A good link to start with that subs usually recommends is: http://www.pneuro.com/publications/dizzy/index.html
Also have a look back through the old post's on this board, they are full of useful info and good links that people have found.
Hope that gives you somewhere to start,