I recommend trying to contact the Department of Taxes to get a email from a sales tax examiner stating that for VT’s purposes the SC IMF is counted as sales tax paid in another jurisdiction. The DMV registers the car, the Dept of Tax is responsible for administration of state taxes. I would assume they would be more knowledgeable about sales tax credits.

If your vehicle was registered for 3 or more years in that state, it shouldn’t matter assuming you can prove that it has been registered in another state for 3 years or more by providing a copy of the out of state title (with a date to back up your claim), 3 years worth of registrations, or a letter from SC’s DMV: https://dmv.vermont.gov/tax-title/vehicle-taxation/exemptions

Best of luck!