- Vue.js

You can embed the Vue js file in your page, add <script></script> and you're ready to get started, no limits every feature is available. Of course for anything other than the most simple stuff you'll want some structure in your app so I use vue-cli which sets up the basic stuff.

- Cloud functions

Can be as simple as writing one in your browser in the azure/GCP/AWS console and as "complex" as using for example the serverless framework to deploy.

That's it!