Deulofeu, recalled to the Hornets' starting line-up after sitting out the FA Cup win at QPR, fired Watford into a first-half lead before scoring twice in as many minutes just after the hour-mark.
Despite their second-half collapse, Bluebirds boss Neil Warnock will no doubt feel that his side should have been awarded a 42nd-minute penalty with the score at 1-0.
Josh Murphy had just made his way into the area when Daryl Janmaat clattered him from behind, but referee Simon Hooper was unmoved to frantic Cardiff appeals as he waved play on.
Cardiff welcomed back captain Sean Morrison after his four-game absence following appendix surgery, while there was a first start for January signing Leandro Bacuna.
Watford announced before kick-off that they had reached an "amicable agreement" with Everton in their dispute over Marco Silva.
The long-running saga had began when Everton targeted then Watford boss Silva following the sacking of Ronald Koeman in October 2017, but the Hornets have long been in a good place under Javi Gracia.
The heady heights of seventh was the prize for a Watford win, and the omens were good with Gracia's men having lost only one of their previous 10 games and fresh from reaching the last eight of the FA Cup.
Early Watford pressure saw Janmaat sidefoot over the crossbar after working a one-two with Deeney and Roberto Pereyra's deflected effort was comfortably held by Neil Etheridge in the Cardiff goal.
Ben Foster had to be sharper at the other end when Oumar Niasse reached Callum Paterson's clever chip over the visitors' defence, the Watford goalkeeper first blocking and then scrambling the ball away to safety.
Two minutes later the visitors were ahead when Morrison, Bruno Manga and Joe Bennett all tried, and failed, to dispossess Deeney and Deulofeu took charge of the loose ball to bury a low shot.
Deeney almost doubled the lead from 20 yards, this time Etheridge getting a hand to the shot, and Warnock was incandescent on the touchline when Murphy was felled and nothing was given.
Cardiff changed formation at half-time with Bacuna and Bennett operating as wing-backs, but Watford still looked the more polished side as Bamba blocked Deeney's goalbound effort.
The energy of substitute Junior Hoilett gave Cardiff brief hope, but Watford soon sprang a counter-attack and Deulofeu beat Manga to run half the length of the field.
Deulofeu rounded Etheridge with ease and two minutes later he burst on to Etienne Capoue's threaded pass to chip the Cardiff goalkeeper.
The Spaniard was not finished there, though, and he skipped through after 73 minutes to provide Deeney with the simplest of tap-ins.
Bamba came up with a consolation by bundling home from a goalmouth scramble eight minutes from time, but Deeney pounced again in the dying moments when Will Hughes set him up from close range.