<div id="login">

    <template v-if="loginType === 'username'">

        <label>Username</label>

        <input placeholder="Enter your username" key="username">

    </template>

    <template v-else="loginType==='email'">

        <label>Email</label>

        <input placeholder="Enter your email address" key="email">

    </template>

    <button v-on:click="typeClick">Toggle login type</button>

</div>

<script>

    var login = new Vue({

        el: "#login",

        data: {

            loginType: "username"

        },

        methods: {

            typeClick: function(){

                return this.loginType === "username"?this.loginType="email":this.loginType="username"

            }

        }

    })

</script>