Vuejs click event from checkbox?

Use @change instead of @click. Click event is triggered before value is really changed.

<input type="checkbox" 
  :value="mainCat.merchantId" 
  id="mainCat.merchantId" 
  v-model="checkedCategories" 
  @change="check($event)"
>

http://jsfiddle.net/eLzavj5f/