It is built on Electron Shell, but vscode is a lot more than a preselected set of plugins for Atom. Most of vscode was written from scratch in TypeScript or assembled from existing (non-Atom) parts. The editors don't really share code.
+1 vscode is just using Electron Shell, not entire Atom. vscode is a well executed project. During initial testing vscode seemed much more responsive than Atom to me.
An Integrated Development Environment as the name suggests integrates all of the tools in a developer's environment: editor, compiler, debugger, source control, servers. An editor is just one part of that.
vscode is intended to be a compiler/debugger/etc. It can already debug node/mono apps. Checkout dnx, .NET Core, and Roslyn. Soon, Visual Studio will not be needed for pure c# applications, except traditional MSBuild/csprojs and C++.