There's a whole site dedicated to VLJ's....
Found this using Google:
http://www.verylightjetmagazine.com/ They are defining it as a jet weighing 10.000lbs or less.
But in general. VLJ's are owner flown, very small jets. Bought as a replacement for Pipers ans Cessnas, both single and multiengine. Cabin space isn't a factor but low aquisition costs and DOC's are. And the reliability of turbine engines as opposed to pistons is definately a big factor.
I think a lot of piston twin owners will take a hard look at a VLJ when considering new aircraft.