The data source of Open vendor invoices form is VendTrans and VendTransOpen. I don't see any data entity that would simply extract information from VendTransOpen table, but it's used in some data entities such as VendPaymentBIVendTransOpen or VendorPaymentJournalLineSettledInvoiceEntity. You can use references to find all such entities. If none of them meets your requirements, you'll have to create a custom entity.