This is right up there with the 'inventor' in the 1930s who developed a carburator that would give every car 200 mpg of gas. It was 'of course' bought up by Big Oil and he was silenced.
Is there vacuum energy in space?
You betcha.
Is this story true?
No way.
If anyone had built a working generator and the Govt had confiscated it, they would be powering every military and space vehicle we have, even if they were kept from the public, which they wouldn't be.
This would be the greatest stimulus to progress since the invention of fire.
No one could succesfully sit on it.
SO9
I'm not wearing tinfoil and I can think of reasons to keep anything of this nature under wraps from a national security standpoint. The best is that you want your opponent to know enough about what you got to deter them from aggression. If deterence fails, you want to have enough in your back pocket to obliterate them. If such a fallback measure depended on something wild like this, you wouldn't want you oppenents to know it is possible beforehand - such knowledge would allow them to duplicate or negate your advantage.
Don't forget the pill you put in the gas tank, so you could fill up with water and use it for fuel.
Of course they could. They've had the Ark of the Covenant crated up in a warehouse for more than 65 years.