vee validate rules
<select ref="country" v-validate :class="{'input': true, 'is-danger': errors.has('required_field') }" name="select_country" >
<option value="US">United States</option>
<option value="OTHER">Other country</option>
</select>
<input v-validate.immediate="'required_if:country'" data-vv-as="state" :class="{'input': true, 'is-danger': errors.has('state_field') }" name="state_field" type="text" placeholder="State">
<span v-show="errors.has('state_field')" class="help is-danger">{{ errors.first('state_field') }}</span>