The centre-back limped out of Villa's 0-0 draw with Crystal Palace on New Year's Day and missed last Sunday's 1-0 FA Cup victory over Blackpool.
Vlaar, who twice underwent knee surgery at previous club Feyenoord, subsequently went for a scan on the issue and he is set for a spell on the sidelines.
"Ron Vlaar expected to be out for four to six weeks with a knee injury," read a post from Villa's official Twitter account.
Manager Paul Lambert also took questions on Thursday about Villa's plans in the transfer market.
The Scot insists the club have received no offers for Vlaar or midfielder Fabian Delph, who have both been linked with exits during the January transfer window.
Lambert did hint at interest in Manchester City winger Scott Sinclair, but would not be drawn on whether Villa would make a move for his services.
"He's a good player, I remember him at Swansea, but I'm conscious he's at another club," said Lambert.
"There will be new players in this month and none now going out."
