When we did it it took a good day and I regret not having the opportunity to be able to stop alon e way. Anyway you pay approx an additional $150ish fot a one way rental from Calgary to Vancouver but I beleive it may be free if you go the other way VanC to Cal (as in no one way surcharge, you still have to pay for the rental). If you need more detail PM me and I'll try and dig up more details. Most Brits/Europeans do it in an westerly direction however doing it the other way saves some of the best bits till last.
I find a car is a better way to get around Vancouver. This is not only because I live here. While the public transportation service Translink bosts that it received the award for the best Transportation service in 1996, I, for one, cannot see how they got this reward. The buses are very slow. If by chance they gain 2 minutes time, they will just stop and wait at a station until they are there at the required time. Bus drivers are very picky and if you have a toe over their imaginary red line (the "line" serperating the front area of the bus from the rest of the passangers seating area) they yell at you. Secondly, there are not many bus routes. I live on a main street and the bus will only come every 20-25 minutes during rush hour, and other routes come only every 40 minutes to 55 minutes. I find that unacceptable for a large city. Also, Vancouver now has the most expensive public transport as they keep raising the prices. This is even more expensive then Toronto (Canada's largest city). This is a shame as many students, seniors and those who cannot afford cars rely on the buses and usualy are living on a fixed income.
Taxi drivers are very expensive so I would also avoid them.
Walking is a great way to see downtown but otherwise I would take a private car as everything is spread out and it will take too long to get around the city using the bus and walking.
The skytrain is a fast way to get downtown but if you are traveling with more than one person, it is more econmical to drive and park downtown as the amount of gas it will cost you will be less than paying for a return ticket for 1 person!. Parking downtown is expensive but not more expensive then Europe of the Middle East. You can also find free parking if you know where to look.
If you plan on hitting the many natural spots of Vancouver (with the exception of the Queen's Park) you will need a truck.