After Ben Foster had made a brilliant double save to deny Oumar Niasse, Deulofeu put Watford ahead on 18 minutes, finishing from a Deeney pass.
On 61 minutes, Watford and Deulofeu had their second of the match. Picking up the ball in his own half, Deulofeu sprinted into the Cardiff penalty area and rounded goalkeeper Neil Etheridge to score.
Deulofeu completed his treble two minutes later, when he was played in by Etienne Capoue and chipped Etheridge.
Deeney had his first in the 73rd minute, while Cardiff pulled one back through Sol Bamba.
The Watford striker grabbed a fifth in stoppage time to send Watford seventh on 40 points. Cardiff are 17th with 25 points.
Cardiff: 26 Feb v Everton (H)
Watford: 27 Feb v Liverpool (A)