Two vendor folders are available in the CakePHP, one folder in the root and another is in the "app" folder.
The root one is the global vendor folder. It is used to place the third-party libraries which are used for multiple applications. The multiple apps share the same root (Cake + plugins + vendors)
The APP one is the app-specific one and takes precedence. It is used to place the third-party libraries which are application-specific.
