Get data from API before app init

This is explained in the Vuejs documentation

https://vuejs.org/v2/guide/instance.html#Lifecycle-Diagram

On your main VueJS instance you need to put your code to query the api in one of the events below e.g.

var vm = new Vue({
    el: '#app',
    data: {
        message: 'Hello World!'
    },
    beforeCreate: function() {
        alert("beforeCreate");
    },
    created: function() {
        alert("created");
    },
    beforeMount: function() {
        alert("beforeMount");
    },
    mounted: function() {
        alert("mounted");
    },
    beforeUpdate: function() {
        alert("beforeUpdate");
    },
    updated: function() {
        alert("updated");
    },
    beforeDestroy: function() {
        alert("beforeDestroy");
    },
    destroyed: function() {
        alert("destroyed");
    }
});

Which exact event will really depend you exact needs but likely to be beforeCreate or beforeMount

@D9705996, thank you for the answer! Could you please tell me, from what event may I work with Vuex? Will it be available on beforeCreate()?

I would perform your api lookup as early as possible and dispatch an action to query your backend

    beforeCreate: function() {
          this.$store.dispatch("init");   
    }

In your store

actions: {
  increment (context) {
       axios.post('/init').then(response => {
          this.$store.commit("initcomplete", response.data.data)   
       }
   }
}

Bear in mind you action is async so you will some v-ifor similar to prevent your application trying go start until your data is ready.

If you have issues just try the next event down I the list in my original post until you find one that works for your needs