"...If I understand the release notes correctly, I don't even need the Cardboard Unity SDK anymore...."
Correct, you don't have to use the Cardboard SDK with Vuforia 5.5, in fact you simply need to:
- set the "Eyewear Type" field to "Video See-Through" , and
- set the "Stereo Camera Config" field to "Cardboard" in the AR Camera inspector panel
and you're all set for Cardboard.
Nevertheless, it is still possible to integrate in the "legacy" way with the Cardboard SDK, like you used to do with Vuforia 5.0.10, if you wish so;
in that case, you can still follow the usual integration guide:
"...is the magnetic trigger on the side of the Cardboard supported by Vuforia alone,..."
Vuforia 5.5.9 exposes an IViewerParameters API which has a ContainsMagnet() method, which is basically used to inform Vuforia about the presence of a magnet in the viewer; however, at this time there is no Vuforia API to detect a magnetic trigger event and/or to register a callback for such type of event.
This type of functionality may be included in a future release, but currently you will need to use a 3rd party libraries (such as the Cardboard SDK itself) or your wown custom implementation in order to detect magnet trigger events.
Feel free to also post this on the Vuforia "Wish List" thread: