npm run dev
[Vue warn]: Failed to mount component: template or render function not defined.
(found in <Users>)
https://forum.vuejs.org/t/split-js-out-from-vue-singlefilecomponent/8418/5?u=9swampy supplied an answer. To be fair there wasn't enough detail in my question without looking at the GitHub repo; bascially I had introduced a naming conflict by not explictly pointing the vue import to users.vue and it was defaulting to users.js instead.
Hence updating App.vue from
import Hello from './components/Hello'
import Hello from './components/Hello.vue'
...fixed the issue.