If you don't mind the gastown/chinatown location, try Boneta great food super good bar.
I don't think you can book thro opentable for large group like that. give them a call and see.
Phone: (604) 684-1844
Good luck! http://www.opentable.com/rest_profile.aspx?rid=19765&hpu=0
Chop is also a great choice. But it's in Richmond. They have a private room next to the lounge but I am not sure if that will fit 20 ppl.
http://www.opentable.com/start.aspx?m=73
click good for groups and see.
I think Sanafir is also a great choice. It's a matter of finding a place still have 20 seats available on Sat nite.