Tom Dele-Bashiru's early penalty and a tap-in not long after from Edo Kayembe turned out to be enough for Watford to inflict a ninth defeat in 12 league matches upon the Hatters, who had beaten the Hornets 3-0 at Kenilworth Road in October.
Manager Matt Bloomfield, lured from Wycombe on 14 January to replace Rob Edwards, has so far overseen three draws and five defeats, with the Hatters seemingly on course to go from the Premier League to League One in two seasons.
Watford reclaimed local bragging rights with a 2-0 derby win that left Luton still rooted to the bottom of the Championship table.
Jones fired a decent Luton chance over early in the second period, with Louza also too high following an intricate move involving Kayembe at the other end.
Kaminski got his next dive right to stop Tom Ince being handed a tap-in and was then behind a shot from Imran Louza, the Watford captain.
