Roy Hodgson said there is still "a long way to go" this season despite claiming his first win as Watford manager.
Villa dominated but Watford grabbed all three points when Emmanuel Dennis powered home Ismaila Sarr's 78th-minute cross after an opportunistic counter-attack.
It ended the visitors' four-game goal drought and lifted them up to 18th in the Premier League, four points from safety.
"The danger is now that we think we've cracked it and that what we needed to do between now and the end of the season has been done," Hodgson told BBC Match of the Day.