In Conclusion: It is not mandatory to install winter tires in the Great Vancouver area by law. However, winter tires are highly recommended to prevent slipping on black ice and hydroplaning during heavy rains.
To answer this question, we need to understand what may cause sliding in winter.
1. hydroplaning: water builds up between tires and ground and lifts the tires.
Vancouver's winter doesn't snow a lot. Instead of snow, most of the precipitation comes from the rain. When the vehicle passes the water peddle, the tire may lose traction with the road due to poor drainage control. Tires with good water-drain ability could pump this excess water away. Water-drain ability depends on tread depth, tread pattern and tire pressure. Be sure to check out your tire before the rainy season.
i. Tread depth: Well-worn tires with tread depth less than 4/32" may need to consider replacing tires to stay safe during raining season. [https://www.tireamerica.com/resource/tire-tread-depth]
ii. Tread pattern: good design has more and larger grooves and more thin slits on the surface of the tread. [https://www.michelinman.com/auto/tips-and-advice/advice-auto/tires-101/how-are-tires-made]
iii. Tire pressure: insufficient tire pressure may result in a concave tire. Concave tires trap water and increase the chance of aquaplaning.
2. Black ice: black ice is thin and transparent; hence it isn't easy to be recognized on the road. When the temperature drops below 0 ℃ in January and February, and if it drizzles, water has a chance to freeze up and turns into black ice. Due to its high transparency, black ice is so difficult to see that drive cannot prepare themselves for driving over it and the potential loss of control, which makes black ice very dangerous. It is usually be found in shaded areas, bridges, overpasses, and intersections.
Therefore, the main reason for installing winter tires is to prevent sliding on black ice. Winter tires with the snowflake logo handle mud and snow well and, therefore, have decent drainage control and wet road grip on rainy days.