I will start off by assuming that the vibration is really happening, if you were to watch the device page States in the upper right corner of this vibration sensor, it reflects the reality of the cat using the box? The sensor is sensitive enough to pick up the cat?
I am not sure Alexa can use Vibration in routines? Last I checked it was only Contact sensors. This might be your issue.
The way I get around this is to use a "device converter" to take any sensor, and make it a contact sensor. I use uDTH Super. [RELEASE] Virtual Switch Universal Device Type (uDTH) - good for Alexa, IFTTT, HSM, and other integrations
