Do you mean : when the use tries to log in or when the user tries to load the login view ?
I don't have this behavior, but you can try this.
As you are using Laravel only for API, you can deactivate the views in the fortify configuration file, it should probably (but not sure) solve your problem.
'views' => false,
But why don't you redirect the logged in user directly from the frontend ? You can define some navigation guards for this.
https://router.vuejs.org/guide/advanced/navigation-guards.html