Visa Merchant Purchase Inquiry (VMPI) is a payment dispute resolution tool.
VMPI allows issuers to communicate with merchants in real time if a cardholder disputes a transaction. The merchant can respond to a VMPI request by either supplying additional transaction information or refunding the transaction in question.
Either action should stop the dispute process and allow the merchant to avoid a chargeback.
More information about VMPI can be found here.