Dim sum on weekday?
By and large yes, especially for the restaurants that do not operate carts. However, the bigger weekend crowds sometimes become the basis for specials that you might not see during the week. Having said that, the big crowds also mean that waiting for a table at the popular spots becomes the norm.
What part of town will you be in, and how long are you staying ?
There's two dominant dim sums ..... traditional Cantonese dim sum (aka "yum chia" in Cantonese or "yeen cha" in Mandarin). This is obviously the most prevalent and is found in many restos all over Greater Vancouver, and like Peter said, throughout the week but selections may be more limited to the standard tried & true dishes.
There's also Northern/Shanghainese dim sum which is a different genre, and fewer restos offer them during the weekdays (that I know of).
Again, where you'll be staying and how/where you can travel to may dictate what's closest to you.