Fresh from ending a run of four league games without a win on Saturday against Sunderland, Walter Mazzarri's men prevailed at Vicarage Road despite having Miguel Britos sent off in the 65th minute.
M'Baye Niang gave the Hornets a 13th-minute lead, the Milan loanee crashing home a superb shot after being freed by Deeney's nod-on from a long ball.
Frenchman Niang then repaid the compliment soon after the break, his long ball being chested down and bundled in by Deeney for his sixth goal in his last eight league games.
After Britos was dismissed, for a second yellow for a foul on Salomon Rondon, Watford had to survive for 25 minutes with 10 men as Albion piled on the pressure but defended stoically to move into the top half of the table.
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(Reporting by Ian Chadband,; Editing by Neville Dalton)
