I can't get any types under UIElements to be picked up by Omnisharp when using VS Code or Vim. I've tried with a minimal project (clean 3d template) with Unity version 2020.3.20f1, Visual Studio Code Editor (Unity package) version 1.2.4, UI Toolkit version 1.0.0-preview.18. I've followed the guide here to get the correct compilation order of packages.
Build still works fine, but the editor's intellisense throws squiggles under all UIElements types (e.g. VisualElement, UIDocument).