Taylor Raddysh scored, Jonathan Toews had an assist, and Arvid Soderblom made 21 saves for the Blackhawks (7-17-4), who have lost five in a row and 13 of their past 14 (1-12-1).
"I thought the first period was really good," Chicago coach Luke Richardson said. "The way we played I thought was a good team effort. We got a lot of pucks behind their [defense]. We didn't get enough to the net, but I liked how we skated and we took the body a little more."
Stephenson gave Vegas a 1-0 lead at 14:41 of the first period on a power play with a one-timer from the right face-off circle on a cross-crease pass from Mark Stone.
Smith extended it to 2-0 with a short-handed goal at 3:56 of the second period, scoring from the high slot after receiving a pass from Karlsson.
"I didn't do anything, 'Karl' did everything by himself," Smith said. "He picked off a pass, created a semi-breakaway for himself and won a puck battle against three guys and was able to find me in the slot."
Kessel made it 3-0 at 8:08 of the third when he received a pass from Daniil Miromanov in front of the net and his shot was carried in by Chicago forward Philipp Kurashev.