Saskatoon's own vacancy picture comes from CMHC's Rental Market Survey, which measures purpose-built rental buildings specifically rather than every empty unit in a city, a different measurement entirely from a raw count of vacant apartments in a market many times Saskatoon's size. For comparison, Saskatoon's resale market currently runs at 1.63 months of supply, a completely different metric describing listings against sales rather than vacancy.
Warehousing, meaning a landlord deliberately keeping a unit empty to avoid rules or push out tenants, is a real phenomenon in some markets, but proving it requires looking at ownership patterns and renovation permits for that specific unit, not simply adding up how many units happen to be empty on a given day.
A single large number tells you nothing about a specific Saskatoon building or landlord, and buyers and sellers here are better served asking about a property's own condition and history than about a statistic from somewhere else entirely. Get your hand-reviewed valuation from Joel Dyck.