Tyler Motte one-timed a pass from Conor Garland to make it 2-1 at 2:38 of the second.
Schenn drove to the net and chipped a shot into the top right corner to make it 3-1 with 29 seconds remaining in the period.
"I just tried to kind of move my feet a little bit, I had a step on [Senators forward Alex Formenton]," Schenn said. "A bit of a chip shot over the glove there. They don't happen very often. I don't know when the last time was that I scored one like that. Usually, I'm checking to see whose shin pad it went off of before it goes in."
Horvat gave the Canucks a 4-1 lead at 4:02 of the third.
Alex Chiasson scored on a rebound at the edge of the crease on the power play to make it 5-1 at 7:38.
Tkachuk tapped in a pass from Victor Mete to cut it to 5-2 at 13:42.
Miller skated with the puck from the Canucks blue line, worked his way past three defenders and tucked it past Gustavsson on the power play at 17:10 for the 6-2 final.
"I think we play hard most nights," Smith said. "Tonight, I thought we played really hard until it got bad, and then we got really frustrated. So, as a coach, all you can do is push the guys to go harder tomorrow."
NOTES: Hughes was plus-3 and had two shots on goal in 21:27 of ice time. … The Canucks went 2-for-6 on the power play; the Senators went 1-for-5. … Vancouver finished 2-3-0 on a five-game road trip. … Senators forward Drake Batherson played his first game since being placed in COVID-19 protocol Nov. 14. He had two shots on goal and three hits in 18:16.