That setting you just asked about is for the Free version of WC Vendors. It will have no impact on Pro. Just leave the text in that field set to “vendors” and ignore the setting forever. 🙂
The post you quoted, is also a quote for WC Vendors Free, not for Pro. In Free, you can use /vendors/username/ or /vendors/store-name/ but since Pro uses custom post types for Vendor Stores, it’s whatever your Permalink is set to.
PS — You might re-save your Permalinks just in case that might be causing the issue. It would be wp-admin > Settings > Permalinks, scroll down, and click Save/Update.