SOURCE

var vm = new Vue({
  el: '#app',
  data: {
    loginType: 'username'
  },
  methods: {
    switchLoginType : function () {
      if( this.loginType == 'username' ) {
        this.loginType = 'email'
      } else {
        this.loginType = 'username'
      }
    }
  }
})

<script src="https://unpkg.com/vue"></script>

<div id="app">
  <template v-if="loginType === 'username'">
    <label>Username: </label>
    <input placeholder="Enter your username" key="username-input">
  </template>
  <template v-else>
    <label>Email: </label>
    <input placeholder="Enter your email address" key="email-input">
  </template>
  <br/>
  <button v-on:click="switchLoginType">切换登录</button>
</div>

body,td,th {color: #DDDDDD}