In 6.3, I saw that structural components are built using vue.js... Is there any documentation regarding what use case in AEM best fits for using vue.js and how to build components using vue
It looks like your question has to do with AEM. I've moved it to the Adobe Experience Manager space.
There is no docs for this JS framework. The recommended way to build components is using HTL. Overview
We have many articles that show you how to get started with HTL.
HTL is server side whereas Vue is client side framework. Both can work together. Moreover I saw that OOTB components in 6.3 being built using Vue, so was curious to know if there is some documentation around it.