A vehicle payment plan agreement is a contract between a buyer and seller of a vehicle that agrees to installment payments. The final hurdle you face is signing the vehicle purchase agreement; a confusing and convoluted document that we explain in detail here.I left with a signed purchase agreement from the dealership. To draft a legal payment agreement for a vehicle sale, it's essential to include specific details and terms to protect both the buyer and the seller. We've put together this guide to simplify car purchase agreements and help you stay protected when purchasing vehicles or selling them. Watch this before you finance your next car (former dealer explains how to read a purchase contract)