SOURCE

new Vue({
    el: "#exercise",
    data: {
        value: ''
    },
    methods: {
        showAlert: function() {
            alert("Alert!");
        },
        saveValue: function(event) {
            this.value = event.target.value;
        }
    }
});

<div id="exercise">
    <div>
        <button v-on:click="showAlert">Show Alert</button>
    </div>
    <div>
        <input type="text" v-on:keydown="saveValue($event)">
        <!-- <input type="text" v-on:keydown="value = $event.target.value"> -->
        <p>{{ value }}</p>
    </div>
    <div>
        <input type="text" v-on:keydown.enter="saveValue($event)">
        <!-- <input type="text" v-on:keydown.enter="value = $event.target.value"> -->
        <p>{{ value }}</p>
    </div>
</div>