So maybe you also ran into the problem that suddenly with Unity 2017.4.0f1 (or any version below the not yet released Unity 2018.1.0f1) version in combination with XCode 9.3 you get a black screen with Vuforia.
You get the following error in the debug log:
<span class="mceItemHidden"><span class="hiddenSpellError">VideoBackgroundConfig</span></span> with screen size of zero received, skipping <span class="hiddenSpellError">config</span> step
To fix that you need to download an older version of XCode like 9.2
and copy the IOS Devicesupport from the new XCode 9.3 to the older XCode 9.2 and build with the older one.
i found an awesome tutorial for that (see below) Thx @Chilton Webb