Some people say, portlet development is out-of-date... but why should you care about that? Maybe you still have an integrated portal solutions like HCL (former IBM) WebSphere or Liferay running, while trying to not loose connection to modern frontend development..
I have found, that especially when it comes to building user-interfaces with a modern user-experience, portlet developers can easily make a paradigm-shift and start migrating towards another architecture with relatively little effort.
This post aims at helping You to integrate Vue.js into your legacy system based on IBM WebSphere Portal Server 8.5, but conceptually it should also work on any other portal framework. You can use it seamlessly in parallel to your existing code for serving Single Page Applications. From there you can move forward.