While part of the initial Vue JS functionality in Moqui Framework 2.1.0 it is only recently well supported and tested so you will need the latest code from the moqui/moqui-framework and moqui/moqui-runtime repositories. This will all be in the upcoming 2.1.1 release series.
Because this is new and many are likely to have questions about it I added an example and some brief documentation that covers the main differences from normal XML Screens. That is now available on moqui.org here:
https://www.moqui.org/m/docs/framework/User+Interface/Client+Rendered+Vue+Screen
To run this locally you will also need the latest code from the moqui/example repository.