[Vuejs]-Vue not defined even though it's been imported

As far as I can tell, you should be using:

import Vue from 'vue';

It might be worthwhile to take a moment to read how the import statement actually works.

What you are doing essentially is importing the module for its side effects only:

Import a module for its side effects only

Import an entire module for side effects only, without importing
anything. This runs the module’s global code, but doesn’t actually
import any values.

import '/modules/my-module.js';

What you want to be doing is import the default export of the module:

Importing defaults

It is possible to have a default export (whether it is an object, a
function, a class, etc.). The import statement may then be used to
import such defaults.

import myDefault from '/modules/my-module.js';