If you visit any vendor, then the SKU Mapping page. You can toggle to unmap products and simply upload a sheet with the product Skus and vendor Skus to unmap those products and send to the default vendor.
You'll need to use this format in your CSV.
CSV File format: ProductSku, VendorSku
ProductSku: This is the SKU from your store
VendorSku: This is the identifier your vendor uses
Example: Bulk SKU Mapping Template