Watford s Nigerian striker Emmanuel Dennis. AFP
Ismaila Sarr chipped a cross to the center of the area and Dennis slipped past substitute Ashley Young before powerfully heading the ball into the net in the 78th minute at Villa Park.
Watford's first league win since beating Manchester United 4-1 on Nov. 20 was also its first under manager Roy Hodgson, who recently replaced Claudio Ranieri.
The victory moved Watford to the top of the relegation zone, though still four points behind 17th-place Newcastle.
The promoted club had gone four league games without scoring.
Steven Gerrard's midtable Villa team has just one win in its last seven league games.
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