The phrase "a vastness of" is correct and usable in written English.
It can be used to describe a large, expansive quality of something, often in a metaphorical or poetic context.
Example: "The vastness of the ocean stretched endlessly before us, a deep blue that seemed to merge with the sky."
Alternatives: "an expanse of" or "a breadth of".
"There's a vastness of space that is crucial for meditation".
Congo is little more than a vastness of rainforest and savannah with a famous river snaking through it.
The images in "Wynken, Blynken & Nod"had made him queasy, testfying to a vastness of activity that went on without him all night.
For the first few years of Robinson's life they lived in the wilds, a vastness of landscape that she is sure "had a huge religious implication for me".
Giant waves that swept away shorefront houses inundated the park's woodland to a depth of perhaps twenty feet and left behind a vastness of shredded plastic in the trees, like the pennants of a cast-of-thousands demon army.
But amid the ramshackle remains of the hospital where the country's most infected patients used to live, Mr. Monfort runs the clinic alone, facing a vastness of unmet need that is as clear as the desperation on the faces around the room.
Marriage itself combines an awesome solemnity – an improbable vastness of commitment – with a playful, tender, intimacy, the small sweetness of the everyday.
SO often in the half-dozen low-budget, impeccably made westerns that Budd Boetticher directed in the late 1950's, the opening shot features a lone horseman approaching the camera out of a lunar vastness of rock.
The scene is a forlorn vastness of yellow-green grass, purplish-gray pavement, regimented saplings, and whitish sky.
By its bold application, astronomers have moved from a limiting one-galaxy universe to an immense vastness of space populated by billions of galaxies, all grander in size and design than the Milky Way Galaxy system was once thought to be.
Then, invariably, we would head out for a late-afternoon hike on the cliffs overlooking the Pacific and sit there for a while, the vastness of the ocean an open platform for our free-floating conversations.
