Thor was helped by the vibrations in the vibranium, but vibranium is just a metal. The power is Thor's. Here is the dialogue to explain what happened:
FRIDAY: The anti-gravs are rigged to flip. Touch them, they'll go full reverse thrust. The city's not coming down slow.
STARK : Spire is vibranium. If I get Thor to hit it...
FRIDAY : It will crack. That's not enough. The impact would still be devastating.
Ultron raised Sokovia into the sky in order for the anti-gravs to flip and go into reverse thrust, sending the city back to Earth to destroy all human life. If Stark gets Thor to hit the spire, the city/land mass will crack but that's not enough because huge chunks will still rain down on Earth and cause devastation.
STARK: Maybe if we can cap the other end. Keep the atomic action doubling back.
FRIDAY: That could vaporize the city & everyone on it..
If Stark blocks the power of Thor's strike on the other end (the bottom of the city because Thor is at the top), then the power from the strike will double back upward and vaporize the city. Without the cap, Thor's strike would have just cracked the city and the power from the strike would dissipate outward into the air after that. Stark creates a cap that reverberates that power back up, preventing that dispersion and making it more effective, but it's still Thor's power.
STARK : I got it. Create a heat seal. I could supercharge the spire from below.
FRIDAY Running numbers
FRIDAY : A heat seal could work with enough power.
STARK: Thor, I got a plan.
The heat seal is the cap Stark creates to bounce the power back up to vaporize the city. Thor hits the vibranium, again which is just metal. The vibrations from the vibranium go downward starting the destruction of the city, hit Stark's heat shield and reverberated back up, completing destruction of the city. It was all Thor channeling the power to destroy the city. Stark merely kept all that energy within the system to maximize its impact.