Driving from banff to vancouver

Great, thanks yes sorry should of said itll be next year, I will be getting a rental so will make sure its a 4x4 :)

My picture of things is that it's, regrettably, unknowable. Winter seems... impatient this year. There are a number of high mountain passes on both logical routes.

September is ok, if you mean December than I suggest you have an 4x4 suv. dec can sometimes be a lot of snow.

I just did this. The conditions can be anything from sunny & clear, to rain, to snow. If you're doing a rental car grab an SUV (all wheel drive). 

September is typically a beautiful drive it is not typical but I have hit snow in the high mountain passes in September.