i took vyvanse 70mg a day for about 2-3 months, i gradually worked up on the dosage and began abusing it heavily for about 3 months taking 210-350 mg a day so i could stay up for 4 or 5 days straight then crash for 2 days. as far as neurotoxic effects, during this time of heavy usage i had some very intense and horrifying auditory and visual hallucinations due to amphetamine psychosis. i eventually went cold turkey off the stuff which felt like hell except i used alcohol to help with the comedowns for the 1st week and a half. after i was free of the worst part of the chemical dependency, i suffered extreme depression, fogginess, and lethargy for a good 6 months as my dopamine receptors had to rejuvenate. i slowly recovered to feeling how i felt before vyvance after 6 months of being clean, but it took a good year before i felt 100% back to normal.
i know that adderall is quite a bit stronger than vyvanse and the fact that you took it for 20 months as opposed to the 5-6 months i took vyvance leads me to believe your going to have a much tougher time kicking this stuff (if that is the course of action you want to take.) you might look into Welbutrin which can function as a kind of recovery drug for people getting off of amphetamines.