Vermont State statutes say that individuals who are required to file Financial Responsibility Insurance with the VT Department of Motor Vehicles must maintain this insurance for a minimum of 3 years. If there is a lapse in this type of insurance coverage the individual is issued a license suspension which would not be reinstated until valid coverage is again on file.
Vermont's Department of Motor Vehicles has very specific requirements regarding the filing of Financial Responsibility Insurance and will accept only certain documents (i.e. an SR-22 certificate from an insurance company NOT from an insurance agent) that are completed in compliance with these strict requirements.
It appears according to the wording of the Vermont statutes on this that you would need the SR-22 certificate for 3 years once your license has been reinstated (not from when you became eligible). You can contact the Vermont Department of Motor Vehicles to double check that this is correct.
Comment Update:An SR-22 is simply a form that is filed with the state that shows you have the coverage that is required to be "financially responsible." The filing of the form is typically an additional $15 to $25. Most drivers experience a higher premium because they have to purchase the additional insurance that makes them financially responsible. In Vermont the minimum required is Bodily Injury and Property Damage Liability, Uninsured and Underinsured Motorist at these limits: 25/50/10.