When I press the assistant button, I just hear "Open the Alexa app and try again." If I unlock my phone and open the app, the button works fine and I can ask Alexa things. But that totally defeats the point. I could always open the app to use Alexa at any time, with any kind of headphones.
I set up Alexa to work with my Bose QC53II headphones.
I set the assistant button to Alexa in the Bose Connect app.
I have the most recent firmware update.
I have added my Bose headphones as a device in the Alexa app.
I have tried clearing the bluetooth devices, turning the headphones on and off again, and updating the firmeware through the desktop program and USB cord.
It works just fine with the Google Assistant, by the way, even if the phone is locked and the app isn't currently open.
To use the action button on the QC35 II headset for VPAs, the app for the VPA needs to be running in the background. It's odd that this isn't the case for Google Assistant unless it's the native VPA on an Android device.