"varies vastly" is correct and usable in written English.
You can use it to indicate that there is a large difference between two or more things. For example: "The amount of rainfall in each region of the country varies vastly."
But although she believes the quality of products on the market varies vastly depending on the quality of their ingredients, the thought of skin creams selling for hundreds of dollars makes her laugh.
The lake occupies a shallow basin almost filled by riverine deposits, and its area varies vastly according to river flow between 283 and 807 square miles (733 and 2,090 square km).
The climate in Brazil varies vastly between different areas.
The results show that the saved energy varies vastly from one user to another.
Thereby, government participation in the housing market varies vastly across OECD countries.
If inclination towards the product category varies vastly across segments, it will be harder to utilize the differences in part worth to offer separating products.
But university systems vary vastly across the Continent.
But the valuation markets put on that flow of profit vary vastly.
From ethics to etiquette, fashions to values, cultural norms vary vastly from place to place – and companies that ignore the differences do so at their own peril.
But since their consequences for health vary vastly from place to place, the differences between those at the top and the bottom can be made smaller.
There is no Europe-wide legislation regulating commerce, and opening hours can vary vastly among countries and among cities within those countries.
