From the National Responsible Fatherhood Clearinghouse:
How to Change Child Support Orders: A state by state guide for to how to change child support orders.
Modify A Child Support Order in Vermont: Once a support order is issued, only the court can change (or modify) it. Either parent can petition the court to modify the order when there is a: Real, substantial and unanticipated change in circumstances (e.g., job loss, significant income change, disability, or change in custody); Need to suspend wage withholding; Change that will result in child support that’s at least 10% higher or lower than ordered; or Court review of their support order, which either parent may request once every three years.
Vermont Office of Child Support Enforcement FAQs and Contact Info