Dimension design idea for vendor and employee relationship

The fact table is invoice_detail, with measures of quantity_shipped, invoice_amt.

Dimensions include part, customer, channel, geography, and vendor (of the part).

In the source system there is this concept of vendor assignment where an employee is assigned to a vendor to fulfill different roles such as the product manager, buyer, invoicing, inventory, etc.

So, an invoice has a vendor for the part ordered. That vendor has multiple employees assigned to it based on a role.

I have modeled a vendor dimension off of the invoice_detail fact table. How do a model the vendor assignment to the employee without snowflaking?

Use another fact table.

A fact table represents a business event or state. Invoicing is a business event. The dimensions relating to the fact should represent those contexts relating to the invoice.

The relationship between vendors and employees is a business state. The nature of that relationship should be reflected in another fact table.

Now, if an employee plays a specific role as it relates to the specific invoice, you can reflect that relationship as a FK to the employee dimension from the invoice.