Sylveon serves both an offensive and defensive Pokemon. It's pure fairy typing only gives it two weaknesses, and a very useful dragon immunity which allows it to not only switch in to them, but counter them easily with STAB Hyper Voice, unlike Vaporeon (water immunity, but no way to scare water types out). It also has access to all the other useful tools that Vaporeon and Umbreon have, such as Wish and Heal Bell. The main downfall for Umbreon is it has a very bad fighting and bug weakness, and with how every Pokemon now gets First Impression, Close Combat, etc. it doesn't perform as well and loses even more momentum by not having offensive pressure. Vaporeon doesn't have too many common weaknesses, but again, doesn't have good offensive pressure.
Hope this helped!