Vettel's victory, at a track where Hamilton has dominated in the past but at which he could manage only fifth place this time, left the quadruple world champions a single point apart after seven races.
Mercedes boss Toto Wolff, whose team still lead the constructors' standings, called it a wake-up call but Hamilton made clear that he was already fully aware of the threat.
"We all know they (Ferrari) have generally had a slightly better package all round, they've been doing a slightly better job so we've got to do more," the Briton told reporters.
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"We've got to keep working, which I know the guys are. And I think we just need to stay positive.
There's no reason to lose control or anything like that, we just need to keep doing what we're doing and keep our heads down and keep motivated and keep pushing because they will falter. We have to keep applying the pressure.
Hamilton struggled with engine temperatures during the race, the Briton already up against it with a power unit on its seventh outing against rivals who brought upgrades to the Circuit Gilles Villeneuve.
Mercedes had planned to have the latest specification on track but delayed it due to quality concerns.
They will need to have that resolved in time for the next race, the French Grand at the Paul Ricard Circuit at Le Castellet next week, a venue that has been off the calendar since 1990 and represents new ground for all the drivers.
"We didn't apply the pressure this weekend, but I'm really going to make sure that we come back strong at the next race," Hamilton added.
The races are coming thick and fast now, with Austria and Britain on successive weekends after France in a triple-header before Germany and Hungary lead into the August break.
Fans can expect a few more twists and turns before then, and hopefully more exciting fare than the last two served up in Montreal and Monaco.
"We came to Montreal expecting to perform strongly and today's result is a further lesson to us that the historic patterns of performance between the teams are not holding true this year," Wolff warned.
"It is very hard to see a trend from circuit to circuit, and the race results are being decided by tight margins.
"Which team makes the fewest mistakes, develops its chassis and power unit most effectively, finds the right balance between performance and reliability, and puts together their race weekend cleanly from Friday onwards."
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