Do I really need the Vive system/ Steam running in order to launch UE4 to start or work on a project?
No you don’t need it. Unless you want to use a (steam) VR headset.
You can disable the Steam VR plugin in your project’s plugin settings if you don’t want it to be used in that project.
If you want to disable it for the engine entirely you could go to to the SteamVR.uplugin file and set “EnabledByDefault” to false. (You can then still manually enable it for VR specific projects)
(this should be the default location)
C:\Program Files\Epic Games\UE_4.25\Engine\Plugins\Runtime\Steam\SteamVR\SteamVR.uplugin