What is the Xero integration with Vtiger CRM? What are the benefits of integrating Vtiger CRM with Xero? How do I install the Xero extension? How do I configure Xero with Vtiger? What modules in Vtiger CRM will sync with Xero? How do records sync between Vtiger CRM and Xero? In how many directions will the records sync between Vtiger and Xero? How do I set up the sync process between Xero and Vtiger CRM? How do I disable the syncing of records between Xero and Vtiger CRM? What fields are mapped for the Organizations module for Vtiger-Xero sync? What fields are mapped for the Products and Services modules in the sync process between Vtiger and  Xero? What fields are mapped in the Invoices module for the Vtiger-Xero integration? What fields need to be mapped in the Payments module for the sync process between Vtiger and Xero? What are the conditions and limitations in the sync process between Vtiger and Xero? What happens during syncing a sales account to an invoice?