Create junction table where one referenced table has a composite key as PK
You have also to define two columns for a double Primary KEY

CREATE TABLE `category_product` (
  `category_title` varchar(20),
    `product_title` varchar(50),
  `product_vendor_name` varchar(50),
  CONSTRAINT `fk_category_product_category_title` FOREIGN KEY (`category_title`) REFERENCES `category` (`title`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `fk_category_product_product_title_vendor_name` FOREIGN KEY (`product_title`,`product_vendor_name`) REFERENCES `product` (`title`,`vendor_name`) ON DELETE CASCADE ON UPDATE CASCADE
);