AP_InvoiceHeader  (this does the details as well).   This imports to the invoice data entry and then you update it.

To expand on what Tom said, be it AP, AR PO, any import of invoices goes into the data entry files and then they must be updated. You only have to use the Header table because VI combines the header and detail tables to present you with one table although you have to indicate the line type on the records tab. If you have never done this before it is advisable to do it with your reseller your first time.

I have actually helped build this integration for a few of my customers. You'll need to write the import for the AP_InvoiceHeader file. Also, what you'll need/want to consider is how to handle invoices going to Credit Card Vendors (Visa, AMEX and Mastercard), versus payments going to an actual employee for reimbursement. We ended up building some logic in ours to handle the vendor assignment within the import based on who was receiving the reimbursment.

You still do load conversions from Concur to Sage?