VueJS accessing a method from another method

I'm using VueJS to make a simple enough resource management game/interface. At the minute I'm looking to activate the roll function every 12.5 seconds and use the result in another function. At the moment though I keep getting the following error:

> Uncaught TypeError: Cannot read property 'roll' of undefined(...)

I have tried:

• app.methods.roll(6);
• app.methods.roll.roll(6);
• roll.roll()
• roll()

but can't seem to access the function. Anyone any ideas how I might achieve this?

methods: {

  // Push responses to inbox.
  say: function say(responseText) {
    console.log(responseText);
    var pushText = responseText;
    this.inbox.push({ text: pushText });
  },

  // Roll for events
  roll: function roll(upper) {
    var randomNumber = Math.floor(Math.random() * 6 * upper) + 1;
    console.log(randomNumber);
    return randomNumber;
  },

  // Initiates passage of time and rolls counters every 5 time units.
  count: function count() {
    function counting() {
      app.town.date += 1;
      app.gameState.roll += 0.2;

      if (app.gameState.roll === 1) {
        var result = app.methods.roll(6);
        app.gameState.roll === 0;
        return result;
      }
    }

    setInterval(counting, 2500);

    ...

    // Activates the roll at times.
  }
}

> You can access these methods directly on the VM instance, or use them in directive expressions. All methods will have their this context automatically bound to the Vue instance.

– Vue API Guide on methods

Within a method on a Vue instance you can access other methods on the instance using this.

var vm = new Vue({
  ...
  methods: {
    methodA() {
      // Method A
    },
    methodB() {
      // Method B

      // Call `methodA` from inside `methodB`
      this.methodA()
    },
  },
  ...
});

To access a method outside of a Vue instance you can assign the instance to a variable (such as vm in the example above) and call the method:

vm.methodA();

You can use vm.methodName();

Example:

let vm = new Vue({
  el: '#app',
  data: {},
  methods: {
    methodA: function () {
      console.log('hello');
    },
    methodB: function () {
      // calling methodA
      vm.methodA();
    }
  },
})

let vm = new Vue({
  el: '#testfunc',
  data:{
    sp1: "Hi I'm textbox1",
    sp2: "Hi I'm textbox2"
  },
  methods:{
    chsp1:function(){
      this.sp1 = "I'm swapped from textbox2"
    },
    chsp2:function(){
      this.sp2 = "I'm swapped from textbox1";
      this.chsp1();
    },
    swapit:function(){
      this.chsp2();
    }
  }
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<div id="testfunc">
  <input type="text" :value="sp1"></span>
  <input type="text" :value="sp2"></span>
  <button @click="swapit()">Swap</button>
</div>