I'm legally banned from Canada and I need to get to Las Vegas. I can't find any direct flights and have to transit through Canada.
Will they arrest me if I land in Canada for a flight transfer?
Let's leave aside the point that there is nowhere in the world where transiting through Canada is your only option for a flight to Vegas.
When transiting to the US through Canada you are not required to clear Canadian customs or immigration. However it is made quite clear that you must comply with immigration and visa restrictions. This means you are not allowed to transit through Canada if you are banned.
You could try this and hope that nobody checks. However if you are not on a single ticket you will have to collect your baggage, which will entail clearing Canadian customs, and thus being denied entry. Also airlines submit passenger lists for various reasons and they are likely to detect a banned passenger. You would almost certainly attract a longer ban and arrest is not out of the question.