The “Analog Hole”
Just as the elaborate DRM (Digital Rights Management, as in consumer-unfriendly copy protection) technology used by the entertainment content industry to protect content from its buyers breaks down at the last point because people actually have to view or listen to content to get any benefit from it (which means that it has to be visible to human analog eyes and audible to human analog ears), there is one obvious way to bypass the content protection given to a message by Vanish: Simply take a screenshot of the decrypted content.
As you will see later in Figure 6, it's perfectly visible, as it must be to be of any use to the user. While you can configure the Firefox plugin to make screenshots impossible, even if it is, no software can prevent a camera from photographing the screen.
It may well be that self-destructing messages always will require trusted users who are actually worthy of trust. The Mission Impossible mission assignment tape could have been run in parallel with an audio recorder to prevent the message's self-destruction. Messages sent on flash paper depend on the reader actually burning it. Even “self-destructing” paper can be photographed before it crumbles, or other means can be found to stop the chemical destruction process from completing.