My co-worker and I were sort of stumped on this. In a last ditch effort to 'try everything' I ran this:
cmd /c --% ""C:\Program Files (x86)\Microsoft Visual Studio\Installer\vs_installer.exe" update --passive --norestart --installpath "C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise"
Notice the mismatched double quotes at the start. This ended up doing exactly what we wanted. Does anyone know why this works?