As best I can reconstruct them from the info posted, you would have these relationsships:
Orders>----X----<Vendor----<InvoiceLineItems--?--InvoiceJoinOrders|------<Orders | Pending_Order Items
I'm not sure I have that correct and am puzzled as to why you have two tables of Line items or are these two occurrences of the same data source table?
And what purpose does the join table serve?
And you have a portal to Vendors placed on your Orders layout...