Vaas, playing for Middlesex, fell as he attempted a difficult catch on the boundary.
The game was held up for more than half an hour while Vaas received medical attention before being taken, on a stretcher but conscious, to hospital for checks.
"Chaminda unfortunately fell back into the boards and hit his head. He's been taken to do some neurological tests," Middlesex coach Richard Pybus told reporters.
"He was conscious and could feel his fingers and feet. You've got to be very careful with an injury like this."
Vaas, 33, had earlier scored 17 not out and taken nought for 46 in eight overs for Middlesex, but his side slumped to a nine-wicket defeat.
