Vokoun turned aside all 28 shots he faced, including 11 in the third period, and finished with a 22-28-5 record for the Panthers, who wrapped up their season in the Eastern Conference cellar at 30-40-12.
Bill Thomas scored the game's only goal with less than 7 minutes remaining in the third period.
Michal Neuvirth absorbed the hard-luck loss, making 22 saves for the Capitals, who clinched the East's top seed Friday with a 4-3 overtime loss against Buffalo.
Washington ended the season at 48-23-11 but failed to match a franchise-best 24th road win. The Capitals will play either the New York Rangers or Carolina Hurricanes in the first round of the playoffs.