You're probably more familiar with these devices than I am (since I'm from So. Cal, and you're from Canada!), but I believe the block heater is actually warming the coolant / antifreeze in the engine block. There are other devices (oil pan heaters, battery blankets, etc.) for warming other under hood parts in cold weather.

With the coolant / antifreeze pre warmed, you should be able to get almost immediate "heat" out the vents (at least warmer than the ambient temperature!).