Christmas Day downtown Vancouver
Are you looking for a more traditional Christmas type of experience or just good food?
Most hotels will have restaurants open. Hawksworth in the Hotel Georgia is new and good. Bacchus in the Wedgewood might have something fairly traditional. We used to have our office lunch there in a private room and they did a good job.
Not sure about Christmas Eve, but in my experience the most dependable downtown option on Christmas Day are the various Japanese izakayas. I recall eating at Guu Garlic on Christmas Day a few years back. I imagine the same would likely be true of the many Chinese restaurants in town. Not sure if any of that fits within your definition of 'nice' or not.
838 Thurlow St, Vancouver, BC V6E1W2, CA