Steam is described as 'Digital distribution, digital rights management, multiplayer and communications platform developed by Valve Corporation. It is used to distribute a wide range of games and related media entirely over the internet, stretching from one-man independent efforts to some of' and is a leading Game Store in the gaming software category. There are more than 50 alternatives to Steam for a variety of platforms, including Web-based, Windows, Android, Linux and Mac apps. The best Steam alternative is GOG.com, which is free. Other great apps like Steam are itch.io, Lutris, Epic Games Store and Ubisoft Connect.
They're both places to get games (and other software) from, but VivePort deals in VR stuff (Vive stuff, even) and is subscription, while Steam is for general desktop gaming and is once-off purchases (excluding DLC).