The odd issue is why the vibration continue after he shut down the engine?

Was it an avionics issue instead? Two previous issues[1] have occurred in the same airspace (near Learmonth) with similar Airbus aircraft from Qantas.

"The incident again fueled media speculation regarding the significance of the Harold E. Holt facility, with the Australian and International Pilots Association calling for commercial aircraft to be barred from the area as a precaution until the events are better understood, while the manager of the facility has claimed that it is "highly, highly unlikely" that any interference has been caused."

[1] https://en.wikipedia.org/wiki/Qantas_Flight_72#Subsequent_Qa...