[Vuejs]-VueJS Validation not firing the first call

So did you tried to call @blur="yourValidationMethod" on your input box inside <template>?

Then define your validation method after data() initialization under <script>

.
.
.
methods:{
          yourValidationMethod(){
            // Your validation codes here, triggered once 'blur' event 
          }
        }

Did tested with simple @blur validation, so it worked.