When you want to add global-level functionalities to Vue, you should generally use mixins or plugins.

For the next examples, I assume you are using vue-cli with the complete webpack template. Moreover, we will use App.vue as a practical reference, but you can apply the same principles to other components...

Mixins

Create a mixin named log.js (in a mixins folder) with the following code:

export default {
  methods: {
    l (...args) { // rest parameters
      console.log(...args) // spread operator
    }
  }
}

Open App.vue, import your mixin and use it:

<script>
  import log from './mixins/log'

  export default {
    name: 'app',
    mixins: [log],
    created () {
      this.l('Foo', 'Bar') // Foo Bar
    }
  }
</script>

Plugins

Create a plugin named log.js (in a plugins folder) with the following code:

export default {
  install (Vue, options) {
    Vue.prototype.$l = console.log.bind(console)
    Vue.l = console.log.bind(console)
  }
}

Open your main.js and declare your global plugin:

import log from './plugins/log'
Vue.use(log)

Open App.vue, import Vue and use your plugin:

<script>
  import Vue from 'vue'

  export default {
    name: 'app',
    created () {
      this.$l('Foo') // Foo
      Vue.l('Bar') // Bar
    }
  }
</script>

You might say: "Hey, why should I have to write this or Vue? I just wanna write l, that's all!". Well... This is actually how Vue has been designed. In order to provide global functionalities (shared by all components), you have to add static properties to the Vue object or prototype properties (Vue.prototype) that are accessible through this in Vue instances.

EDIT

I have just thought about an alternative solution...

You can edit your index.html to add this:

<script>
  var l = console.log.bind(console)
</script>

Then, to avoid ESLint errors, you should also edit your .eslintrc.js file to reference your new global variable:

globals: {
  l: true
}

The file looks like this:

// http://eslint.org/docs/user-guide/configuring

module.exports = {
  root: true,
  parser: 'babel-eslint',
  parserOptions: {
    sourceType: 'module'
  },
  globals: {
    l: true
  },
  env: {
    browser: true,
  },
  // https://github.com/feross/standard/blob/master/RULES.md#javascript-standard-style
  extends: 'standard',
  // required to lint *.vue files
  plugins: [
    'html'
  ],
  // add your custom rules here
  'rules': {
    // allow paren-less arrow functions
    'arrow-parens': 0,
    // allow async-await
    'generator-star-spacing': 0,
    // allow debugger during development
    'no-debugger': process.env.NODE_ENV === 'production' ? 2 : 0
  }
}

Restart your dev server. Now you should be able to use l in your code:

<script>
  export default {
    name: 'app',
    created () {
      l('It works!')
    }
  }
</script>