Vue.js on:change event

Vue on change event  will be fired when you change the data in your html controls like select or input

So you can add the textChangeHandler method to your control as shown below.

<input  :id="product.id" v-on:change="textChangeHandler($event)" v-model='purchaseorder.products[index]' />

Now you can define your handler like below

 methods:{
    textChangeHandler:function($event){
       console.log($event.target.id)
     });
  }

Here you can simply capture the id of the control when user change the value of the input control

Vue change events of the other HTML controls also work in same way.