<input type="text" name="" placeholder="请输入用户名" id="userName" >
 <button class="disabled" id="login">登录</button>

 .disabled {
  pointer-events: none;//禁用点击事件
  cursor: default;//鼠标禁用
  opacity: 0.4;
  }

//监听input里的值
$('#userName').on('input propertychange',function(){
   if(this.value.length != 0){
      $('#login').removeClass('disabled');
   }else{
      $('#login').addClass('disabled');
   }
 });

<template>
  <div>
       <input type="text" placeholder="请输入用户名" v-model="userName"> 
      <button :disabled="forbidden" >登录</button>  
  </div>
</template>

export default{
  data(){
     return{
       forbidden:false,
       userName:null
     }
  },
  methods:{
     if(this.userName == null){
       this.forbidden = true;
     }else{
       this.forbidden = false
     }
  }
}