I tried my usual "Simpsons, Plex" yesterday and I got a "I can't find The Simpsons on Plex On Vue right now".
So I exited the Vue app and tried again. Again no luck. I even tried just "The Simpsons" which normally brings up Plex as one of the options, but it's gone now. But they did add support for Vue, so that's something, right?
It should work via google assistant on the Shield. I know that was included in one of the recent updates.