What is the correct way to use vue js when connected as a module?

Question

Vue.js

There is a site. I'm trying to figure out how to properly connect vue js.

1. Use the root element as id.
It turns out that in this case, vue will be able to manage all the elements. I won't have much vue logic. Is it necessary or is it better to narrow the vue connection to a specific area

2. If I connect to the root element, if there is javascript in the layout, I get a template compilation error. How to get around this limitation?

3. If I use several Vue elements, then how are computed values ​​made if they depend on values ​​that are in different vue ?

I would be grateful if you explain.
Vue js just learning