"vastly benefit" is a correct and usable phrase in written English.
It is used to emphasize the degree of benefit. For example: "The new training program will vastly benefit the employees by increasing their skillsets."
Moscow has a $3.5 billion, 23-year agreement with Baghdad to rehabilitate several Iraqi fields that would vastly benefit a Russian oil consortium led by LukOil, they said.
The well-being of our own country, too, would vastly benefit from a 25 percent reduction in the military budget, releasing urgently needed funds for use among our own people.
Progresses in the field of rational drug design and computational chemistry will vastly benefit from the availability of increasing structural knowledge of both the kinase domains and the ligand-binding sites of these receptors.
Newspaper interviews and other speaking opportunities were strategically used to carefully shape the public perception that a cut in alcohol taxation would vastly benefit consumers through a reduction in retail prices.
I believe that the paper would vastly benefit if one specific case was picked, for which the practical approach of the Butterfly model would be elaborated, and the benefits would be highlighted in a very concrete way.
Argentina has regained its prosperity partly out of dumb luck: a commodity price boom has vastly benefitted this soy, corn and wheat producer.
The fight against many diseases, including HIV/AIDS has vastly benefited from the UN's leadership, diplomatic and/or operational.
A modern driver, in other words, gets vastly more benefit from a gallon of gasoline — makes far more economical use of fuel — than any Model T owner ever did.
The rules, intended to benefit consumers, vastly expand federal authority to direct the use of premiums collected by companies like Aetna, Humana, UnitedHealth and WellPoint.
On one side are those who benefit from vastly improved access to technology and capital -- which is to say, workers in developing countries.
Some can benefit from vastly different constituencies becoming bedfellows and dreaming the same dream.
