Vendors are usually synced from your connected inventory management system. If you need to remove a vendor that has originated from your IMS, you should remove it in your IMS directly.

Otherwise, you can remove a vendor in Inventory Planner by going to Catalog > Vendors.

Click "Settings" beneath the vendor you want to remove.

Then click "Remove Vendor" at the bottom of the screen.

Then click "Yes" to confirm you want to delete the vendor.