UserWebComponent:

<template>
  <div>
    <h1>My Vue Web Component</h1>
    <div>{{ msg }}</div>
  </div>
</template>
<script>
  export default {
    props: ['msg'] 
  }
</script>

$ vue-cli-service build --target wc --name user-component  --mode production ./src/UserComponent.vue

<script src="https://unpkg.com/vue"></script>
<script src="path/to/user-component.js"></script>
<!-- use in plain HTML, or in any other framework -->
<user-component msg="probando"></user-component>

• https://cli.vuejs.org/guide/build-targets.html#web-component

vue-cli-service build --target wc --name foo 'src/components/*.vue'

Los componentes se inferirán a partir del nombre. foo-user-component por ejemplo para UserComponent.

Algo parecido a esto:

import Vue from 'vue';
import wrap from '@vue/web-component-wrapper';
import VueWebComponent from './components/VueWebComponent';
const CustomElement = wrap(Vue, VueWebComponent);
window.customElements.define('my-custom-element', CustomElement);