"vast city" is a correct and usable phrase in written English.
You can use it to refer to a large city, for example: "Tokyo is a vast city, with its population of over 13 million people."
Tenochtitlan was a vast city with great monuments.
The line also travels north, south, east and west in the vast city of 23 million.
Dark smoke billowed over this vast city in the late afternoon.
At the vast city dump at Ojota, where thousands eke out a living, shacks are cleared out frequently, residents complained.
Then he dumped the body in the River Tiete, which crosses Sao Paulo state, regularly flooding this vast city.
Since then there have been revelations of fiscal tricks whereby vast city deficits were concealed by past or present administrations.
Hernández moved in with a family friend while she searched desperately for another job in the vast city.
And they are yearning for connection in a vast city that can silence the most gregarious extrovert.
They also look like little buildings, and the whole resembles a vast city on a steep hillside.
As São Paulo developed into a South American business hub, though, it transformed into a vast city, a labyrinth of concrete buildings and tangled streets.
In one project, which has engrossed him since 1972, he has been carving a vast "city" into the desert of Nevada (without allowing anybody to see it).
