If you are setting up workflow to approve vendor invoices you might want to consider if you have approval at the invoice header level or at the line level. The line level would allow you to have multiple approvers for the different lines of a vendors invoice if they needed to be split out for example if the vendor provided one invoice for multiple  purchase orders. There is a workflow for this which is the vendor invoice line workflow.

Here is a quick overview.

AX2012 R3 CU11

Cheers

Lachlan