Thirty-one units were damaged in the fire that broke out on Keele Street, south of Major Mackenzie Drive West.
There were no serious injuries.
Vaughan Fire Chief John Caruso told CBC News that it is too soon to say whether the fire was started intentionally.
For now, investigators are looking at either an electrical failure or a cigarette as the possible cause.
The damage is estimated at $4 to $5 million.
Some residents could be forced out of their homes for months as a result of the damage.
It's the second time fire crews have been called to the same location in recent years.
The previous fire in 2009 left some residents without a home for up to a year.