It was the first multi-goal game for Ovechkin since Oct. 29 and the Capitals have won back-to-back games for only the third time in two months.
Ovechkin scored just 1:42 into the game while Buffalo's Mike Weber was in the penalty box for boarding.
Washington was protecting a 2-1 lead when Ovechkin put a slap shot past Ryan Miller with 5:50 to go in the third. Ovechkin has 14 goals for the season.
The other goal for the Capitals came from Nicklas Backstrom 3:52 into the second that gave Washington a 2-0 lead.
Vokoun allowed only a second-period goal by Jochen Hecht. Miller surrendered the three goals on 21 shots.