A vehicle payment plan agreement is a contract between a buyer and seller of a vehicle that agrees to installment payments. This document outlines the terms of an owner-financed mortgage between a buyer and seller of a property.Download and print free vehicle purchase agreement templates. Ensure a smooth car buying process with these contract templates. A payment plan agreement outlines an installment plan to repay an outstanding balance over a specified time frame. How to fill out the Installment Purchase Agreement Template for Users? 1. Read through the agreement thoroughly. 2.