Infantile spasms or West Syndrome, as in your child's case, is a devastating epileptic disorder of infancy in which less than 25% children have a good prognosis.
In case your child has had no fits (jerks, head drops, frequent eye blinking, blank spells etc) for 2 years, you should get a sleep EEG done. If the EEG is normal, one can do a gradual withdrawal of medicines.
If the EEG shows epileptiform activity, then there is a high chance of relapse of fits on stopping medicines.
Further, since your child has been on Vigabatrin for a long time, a change of this drug to another should be considered because about 30% of patients who receive this medicine for more than 1 year develop retinal/ optic nerve damage.