Using Vanced itself is not necessarily illegal, but it may violate YouTube's Terms of Service. Vanced is a modified version of the official YouTube app that allows users to view ad-free videos and bypass certain restrictions, such as playing videos in the background. However, YouTube's terms prohibit users from modifying or reverse engineering its services. This means that using Vanced may result in your account being terminated or your access to YouTube being restricted. It is important to carefully consider the potential consequences before using any modified version of an app.