Matthew Noad, 30, and Clive Griston, 52, took luxury holidays in the Caribbean and dined in expensive restaurants with life savings they had scammed from elderly investors.
They cold-called scores of vulnerable victims promising amazing returns if they bought land in Scotland, Devon and Northamptonshire.
Investors were told it would rocket in value once it was developed but none of the land was eligible to be built on and their money has never been recovered.
One of the victims, a blind 89-year-old man from Cardiff, was reported to have paid £94,000 for worthless plots of greenbelt land in Dumbarton.
The pair were jailed for four years and eight months each at the Old Bailey.
Judge Christopher Moss said: "Your victims were just about as vulnerable as could be imagined, most were elderly people who were completely taken in by your sophisticated sales platform."
Judge Moss said the scam was "skillfully" planned and "ruthlessly conducted" with a total loss of around £10m.
"It's difficult to imagine a more cynical dishonest way to effectively steal hard-earned money from victims," he added.
Griston, of Larch Dene, Orpington, Kent, and Noad, of Forest Drive, Keston, Kent, admitted making false representations about the financial viability of land in Dumbarton, Cullompton, Devon and Towcester, Northants, from December 31, 2005, to June 23, 2011.