Edit in JSFiddle

var MyCmp = {
  template: '<div style="padding: 10px; border: 1px solid red; background: white;">\
    <slot></slot>\
  </div>'
};

new Vue({
  el: '#app',
  components: {
    myCmp: MyCmp
  }
});
<script src='https://unpkg.com/vue@2.4.4/dist/vue.js'></script>

<div id='app'>
  <my-cmp>
    <h3>Hello from slot</h3> 
  </my-cmp>
</div>