This functionality is new as of version 4.1.
While adding Reorder Items to a 'New' Product Reorder, if a Product is not linked to a Vendor, it can be linked via the 'Add / Link Product to Vendor' dialog.
The 'Add / Link Product to Vendor' dialog is a reduced version of the 'Product Info' screen, and will capture the minimum amount of information needed to link an Existing Product and Vendor.
To Link a Reorder Item to a Vendor: