Stop leak damage aside and not sure what year your car is, this is perfectly normal for a Vibe in -25c. If this coincidentally began as soon as the temperatures dropped into the deep freeze, read on..
Being in QC, I've experienced this on my car pretty much every winter a few times with extreme cold. The 1.8 takes a long time to heat up and the 2009-2010 Vibes have way too much grill ventilation which leads to convection heat loss once you get on the highway. There is a reason you see every tractor trailor with a complete grill block in the winter in these parts. If you're engine hasn't been running for more than 15-20 minutes it is not fully warm in -25c, and you have your heater full hot, your fan is on high speed and you don't have a grill block, your coolant temp will indeed drop and the air will start getting cooler.
Steps to combat this...
1) Do a grill block of the entire front end, both kidneys and the inlet below.
2) Let the car idle for 2-3 minutes before you drive off when it is that cold.
3) Keep the fan off and resist the urge to turn it on until the coolant temp gauge is close to the middle. When it gets there, only turn it up to 2
4) If you're running the original factory coolant you get a 50/50 mix on the assemble line. Toyota offers a 55/45 mixture for our cold climate. Get a flush done at a Toyota dealer or DIY using Toyota Super Long Life 55/45. Toyota charges less than $100.
Also, I had the upgraded Denso starter (28100-0T051) installed as my OEM starter was grinding in the cold weather. Amazon sells them for a very reasonable price. I highly recommend anyone in very cold climate gets the upgrade. It is absolutely night and day in terms of cranking speed. Barely turns over and fires on the spot even in -25c..
2010 Red Hot Metallic 1SA