I have purchased arrival and departure airline tickets, but no hotels yet.
To add extra flexibility to stay in each city more or less time should it be safe to just show up without hotel reservations and get last minute room deals via Hoteltonight, Booking.com or Priceline?

I have a place to stay in Victoria, so don't know how easy or hard it is to find a place, but your timeframe is definitely not high season there. It will be close to Whistler, as IIRC skiing season will have started by then. Vancouver, never had a problem finding a place, but always looked close to YVR for early departures or very late arrivals.

While in Victoria an interesting area to look around and shop is Lower Johnson Street and Market Square. A lot of small independent shops and restaurants in mid 1800 brick buildings. If you walk one block further north you can go through Fan Tan alley to Chinatown. Basically one street but also small shops with some non-Chinese as well.

Previous posters have comment saying to skip Butchart Gardens on this trip which is good advice with your limited time on Victoria.
Regarding Helijet check with the hotel you are staying in as some of them have shuttle service to there. It is only a short cab ride, 2 kilometers, if you have to take one.

I looked at the Helijet terminal in Vancouver on Google Maps and it seems to be in a very "awkward" to reach location. It is close to town, but a bit far to walk especially if the weather is not good and you are carrying bags, but at the same time 2km is a very short cab ride that the drivers may not be interested in. There is no convenient public transit close by either. So, it seems like there should be a shuttle provided by Helijet to pick up and drop off passengers at least to a central point downtown.

The cab drivers will have no issue whatsoever with this journey. They are very used to short trips around the downtown core.

If you want to walk, enter Waterfront Station, follow the enclosed footbridge over the rail yards, go down the escalator, and follow the "helicopter" signs outside to east parking lot and the heliport. Once outside, it's about a five-minute walk, give or take.

https://www.google.ca/maps/@49.28642...!3m1!1e3?hl=en

Here is a Google Street View picture of the door you will exit from waterfront station. Twirl around and double click your way through the parking lot and you'll see the path to the heliport

https://www.google.ca/maps/@49.28649...2!8i6656?hl=en

I have seen the Helijet shuttle vans around town, although I've never taken one as getting to the terminal is very easy.

tripweb:
Just to be clear the Helijet terminal in Victoria is 2km from downtown while the terminal in Vancouver is in a bit of an out of the way place. If you don't have too much to carry and it's not raining when you arrive in Vancouver then the instructions above are good, except in the opposite order as you are arriving not departing. If you decide a cab would be a better choice when you arrive in Vancouver you can advise Helijet of this when you check in at the Victoria terminal and they will make sure a cab is available for you as you arrive. Many of the passengers do this so it is common practice.
Have a good trip.

With respect to Butchart Gardens, December is one of the most popular times to visit, because they have fabulous displays of lights around a 12 days of Christmas theme. Evening is the time you want to go(it's open until 9) , so if it fits into your schedule, it's well worth it.

The website is http://www.butchartgardens.com/activ...ristmasSection