The precise answer to your question is actually “both” and “neither”.
In all cases, regardless of what technologies you use, the “front-end” is a combination of HTML and JavaScript that runs in the browser.
Django works by creating (as necessary) and sending HTML from the server to the browser. That HTML may include or contain references to other files containing JavaScript.
When you use something like Vue (etc), you’re adding Vue’s JavaScript to the response you are sending from the server. Those Vue modules are then doing work for you that you may have otherwise had to do yourself in Django (or write your own JavaScript to do the same things).
The question isn’t “what is the front-end using”, it’s “what is generating the html that the user is seeing”. When you’re creating the front end only using Django, you and Django are creating all the HTML that will eventually be seen by the user. When you add something like Vue into the mix, you’re adding another program that will generate HTML - affecting what is visible in the browser.
Therefore, if you’re looking for “easiest to learn”, starting out with just Django is the best place to start. You can add features and functionality later as desired.