Can I depart from Banff and arrive at Vancouver on the same day ? And how?
Well, Canada is a huge country. And you are talking about distance of 850km that take at least 10 hours to drive.
If you do not drive, I guess, it leaves you with 2 options. Plane from Calgary, and I do not think it is what you are looking for, or a bus.
Try to check Greyhound. They have bus services.
Or you might try to hitchhike. With a bit of luck you might be able to get to Vancouver in one day.
Flying from Calgary to Vancouver would take about an hour where as taking a bus from Banff to Vancouver would take about 10 hours.
I advise flying as it takes much less time and does not cost a whole lot more money.