Dimensional modeling of product and vendor for invoice fact

On the source system, the invoice_detail table carries the both the part_id and the vendor_id (for that part) on it.

The source system part table carries the vendor that supplies that part. A part has only one vendor.

Is there any value to having two dimensions, part_dim and vendor_dim to join to the invoice_detail fact table?

You are far better off with two dimensions. Each dimension table will be smaller and perform better. Plus you can implement the relationships (per your other post) between vendor and employees. If it was one dimension, you would need to relate employess to parts... probably not a desireable thing to do.

It also gives you greater flexibility should the business processes change... such a sourcing the same part from multiple vendors.