export default myHelper {
    myHelperFunction(param) {
        return param;
    }
}

<template>
  <v-text-field :rules="[myRule]"></v-text-field>
</template>

<script>
import myHelper from './myHelper.js';

export default {
  name: 'MyComponent',
  data() {
    return {
      myCls: new myHelper(),
      myRule: this.callHelperFunction,
    };
  },
  components: {
    myHelper,
  },
  mounted() {
    this.myCls.myHelperFunction();
  },
  methods: {
    callHelperFunction(param) {
      this.myCls.myHelperFunction(param);
    }
  },
};
</script>