Unfortunately, since you already have a Laravel application, you cannot just install Vanguard over it. Vanguard is originally designed to be used as a foundation for a Laravel project you want to build.
So, if you want to integrate Vanguard into an existing Laravel app, the only way to do it is to copy the files manually. Or, if your application is small, to copy your application files to Vanguard. I would recommend you to start with routes and controllers and then continue with the views and assets.
The way I would do it is by moving your application files to Vanguard. You can group Vanguard related routes with a "/admin" prefix for example, and then use your routes the same way you use them in your app now.