If you are rated 100% disabled according to the rating schedule soley for a mental disability and you are working, then VA will probably want to take a look at you once they catch wind of you being able to work. This is because severe occupational impairment is a portion of the 100% rating criteria for mental disorders. Otherwise, if you're rated for any other disability or combination of disabities at the 100% rate , then you can work as much as you like without VA caring.
If you're rated at less than 100% but receiving the 100% rate due to Individual Unemployability (IU), then this will generally exclude from one working. If one does work while on IU and the VA catches wind of it, they'll more than likely schedule you for an exam and in the end probably take away the 100% disability. You would then go back to your regular disability rating that you had before the IU was awarded.