<div id="exercise">
    <div><button class="test" v-on:click="newCheck()">New Challenge</button>
        <p >Success check is: {{ passCheck }}</p>
    </div>
    <div>
        <button class="roll" v-on:click="roll20()">Roll!</button>
        <p>You Rolled a {{ roll20() }}</p>
    </div>
</div>

new Vue({
    el: '#exercise',
    data: {
        yourRoll: '10',
        passCheck: '10',
    },
    methods: {
        roll20: function(){
            this.yourRoll = Math.round(Math.random()*19)+1;
            return this.yourRoll;
        },
        newCheck: function(){
            this.passCheck = Math.round(Math.random()*19)+1;
        }
    }
});