The Brewers have won two out of three in the four-game series.
Aoki broke a scoreless tie with a two-run single in the fifth and Schafer added a two-run triple one inning later.
The lone Washington run came on a homer by Anthony Rendon in the seventh, his second of the season.
Kyle Lohse (5-6) worked eight innings to get the win, giving up a run on four hits with seven strikeouts.
Francisco Rodriguez came on to nail down the victory, but he gave up a single and walked a batter before getting Rendon on a fly ball to deep center that ended the game. It was the seventh save for Rodriguez.
Two of the runs allowed by Washington starter Ross Detwiler (2-7) were unearned on eight hits in a six-inning effort.