One angle on this - if you are making a compelling App with Vue - to do anything really interesting you will be talking to an API (using Axios).
Those back-end database tasks - can be done very efficiently with Codeigniter. This is where CI's low footprint, fast setup, and blazing fast performance really are a huge advantage to getting something up and running fast.
And then if you are hosting on a PHP server - no build tools, no cli, no webpack needed. You can develop a fully working Vue app much more quickly.
Of course you can tell your clients - this is a Vue powered App hurrah! Which is true - Vue runs in the browser - it can be hosted on anything.
And then even if the main website framework is not PHP or Codeigniter - the Vue powered improvements/new features that will be inserted into the existing framework - those API endpoints can be served by Codeigniter. I know this is happening from other developers i interact with. They are not going to publicly say they are using CI - they talk about the *hot* tech like Vue - but they are still using CI and other PHP frameworks for the 'shell' that hosts the Vue app, or just the API, etc.
