Sports bars have not been so good in downtown Vancouver over the years. Right downtown, a decent bet is the Lennox Pub, at the Corner of Granville and Robson. You can call and see if the sound will be on for the Seahawks' game. They have many screens, OK beers and decent food.
The Donnelly Group runs a number of bars that have good beer and food and many screens.
They have a newer one in an old bar location now called the Butcher and Bullock at 911 Pender. I haven't been there since it changed ownership to the Donnelly Group. It was a very spacious dump with bad reception screens and warm beer and bad sight lines for the TV's, but the new owners run the best string of bars in Vancouver, all with screens, decent craft beers and good food. They advertise NFL Sundays on line at this location, so this might be a very good bet. Hopefully they have improved the screens, etc. Others in the chain include The Lamplighter in Gastown, the Cinema Public House on Granville and Library Square at the (duh) Library on Georgia Street.
If you like outstanding craft beer and want to venture out of the downtown core, I recommend the Tap and Barrel in Olympic Village, on Athletes' Way. Very nice, new place, right on the water with great views. Sit at the bar downstairs. The bartenders are all Hawks' fans. No sound, though. Poutine is excellent, as are the pizzas.
Another option, where sound might be on, is Original Joe's, at the corner of Cambie and Broadway. The usual bartender is a whiners' fan, so the sound is generally on for their games instead of the Hawks' games, and that's why I boycott that place.
Other options are the bar side of the Rogue on Broadway (two blocks west of Cambie) and, a little farther west, the Academic on Broadway (at Fir), which is a block west of Granville). The Academic might be empty on Sunday PM, in which case they'll put the sound on. It is also a Donnelly Group bar. They have yet another location on West 4th Ave. called Bimini's.
You probably won't find a bar with a dozen fans in Hawks' jerseys. And you probably won't find a bar that will have the sound on for your game unless you can talk them into it.