Check it out yourself at: https://yithemes.com/themes/plugins/yith-woocommerce-multi-vendor/ and read the terms and conditions:
“Our products are created for WordPress, an open source software released under the GNU public license http://www.gnu.org/licenses/gpl.html . Therefore, any part of our products which constitute a derivative work of WordPress are also licensed under the GPL 3.0. Our prodocts are comprised of several different file types, including: PHP, Cascading Style Sheets, JavaScript, as well as several image types including GIF, JPEG, and PNG. All .php and .js files are released under the GPL 3.0 unless specified as otherwise within the file itself. If specified as otherwise, the files are licensed (or dual licensed, as stated in the file) under the MIT License http://www.opensource.org/licenses/mit-license.php, a compatible GPL license.”