"vast source" is perfectly correct and usable in written English.
You can use it when referring to a large and expansive source of information, knowledge, resources, etc. For example, "The internet is a vast source of knowledge."
The second edition of Holinshed's Chronicles was at hand as a vast source for Shakespeare's historical playwriting.
British to its core, it is a vast source of all those half-forgotten rhymes for counting, patting, jiggling and soothing babies and toddlers.
He was prospecting for a vast source of clean, renewable energy that seemed straight out of a Jules Verne novel: the heat simmering within the earth's bedrock.
This number represents a vast source of untapped income – and in an era of declining retail sales for games, identifying new streams of digital revenue is becoming vitally important.
It doesn't help, Mr. Gurría said, that the United States, the world's second-largest energy consumer after China, has found a vast source of oil and natural gas in shale rock.
This exercise doesn't really need to have anything to do with social media, however, I do think that the internet is a vast source of information on careers, top professionals in each industry and the top jobs/employers in your field.
This is the new world of big data and insurance, where social media websites are vast sources of valuable information just waiting to be plundered.
From this it followed, Dr. Gold argued, that oil wells might be naturally replenished from vast sources of carbon deep in the planet.
"The two biggest clubs in the land with the two best fan bases in the land have been sold down the river to a bunch of no-mark Yanks, intent on stealing money from our vast sources of revenue.
Shale gas has come under the spotlight as vast sources in the US have proved a cheap source of fuel there, but allegations of widespread pollution caused by the shale gas projects has led green groups to call for a moratorium.
Many of the Internet's gains are fun — games, chat rooms, Twitter streams — rather than vast sources of revenue, and when there have been measurable monetary gains, they often have been concentrated among a small number of company founders, as with, say, Facebook.
