vue-loader: Module build failed: TypeError: this._init is not a function

I’m trying to build an electron app that will be using vue.js as well as electron-builder for final distribution. The electron-builder docs suggest using a two-package.json setup where the devDependencies are in the main package.json, and the dependencies are in app/package.json.

However, when I try to incorporate vue.js into this app, I run into problems because of this two-package.json setup. I get the following output when trying to run webpack:

[Vue warn]: Vue is a constructor and should be called with the new keyword 
Hash: a745e98e5605ad253bf8
Version: webpack 1.13.2
Time: 704ms
    Asset    Size  Chunks             Chunk Names
bundle.js  152 kB       0  [emitted]  main
    + 3 hidden modules

ERROR in ./app/components/MainScreen.vue
Module build failed: TypeError: this._init is not a function
    at Object.Vue$2 (/Users/rod/Innovative/projects/aglist/kiosk/varius-kiosk/app/node_modules/vue/dist/vue.common.js:2594:8)
 @ ./app/index.js 7:18-56

My index.js looks like this:

import Vue from 'vue'
import MainScreen from './components/MainScreen.vue'

new Vue({
    el: 'body',
    components: { MainScreen }
})

And the directory structure of the relevant files in my project looks like this:

/package.json (defines devDependencies)
/webpack.config.json  (uses app/index.js as the entry)
/app/package.json (defines dependencies)
/app/index.js

Commenting out the import MainScreen line results in webpack running successfully. Another configuration where webpack works is if I move vue from a dependency in app/package.json to the main package.json, and also remove vue from the app/node_modules folder. This is not acceptable, though, because I need vue as a dependency in app/package.json in order to do make my app installers. If I have vue as a dependency in both package.json and app/package.json I still get the error above, even if I explicitly import Vue from '../node_modules/vue' in index.js. (IE, webpack seems to be using /app/node_modules/vue rather than /node_modules/vue)

Am I missing something here? Could anyone help me configure this so I can use both vue.js with webpack, and electron-builder with its two-package.json setup?

This issue is related to #171 which was closed due to inactivity.