Graham took his tally for the season to 12 with goals either side of Helguson's strike as the Hornets grabbed the victory they needed to be certain of beating the drop.
Instead, Crystal Palace and Sheffield Wednesday will meet next Sunday with one of them condemned to slipping into League One.
The Hornets have endured a tough season, during which they have had to sell their key players and narrowly avoided going into administration.
But they can relax as the final-day drama unfolds after coming up against a Reading side in end-of-season mode after their play-off hopes were ended in midweek.
Watford kicked off on the back of a disastrous run of two wins from 15 games while Reading had lost just three of their previous 19 matches.
But the form book meant little as the Hornets flew out of the blocks and fully deserved their lead, which arrived on the half-hour mark, having dominated the opening stages.
John Eustace headed Henri Lansbury's free-kick wide and Ryan Bertrand hacked a goalbound Martin Taylor volley clear before Graham sent the home crowd wild.
Lee Hodson sent a long throw into the area, Helguson flicked the ball on and Graham arrived at the far post to nod past Adam Federici.
The hosts went searching for a second and Arsenal loanee Lansbury's free-kick curled the wrong side of the post before Helguson's snap shot flew narrowly wide
Lacklustre Reading had only a Shane Long header, which drifted wide, to show for their first-half efforts.
Watford did not have to wait long after the interval to double their lead, with Helguson firing his 11th of the season in style.
The Icelandic striker, who is on loan from QPR, controlled Don Cowie's cross in an instant and lashed his shot past Federici two minutes into the second half.
Seven minutes later Graham wrapped up the win when he latched onto a mistake by Andy Griffin, burst through and tucked the ball past Federici.
Graham could have completed a memorable hat-trick when he met Helguson's cross on the penalty spot but he blazed his first-time shot over.
But it mattered little as the final whistle blew with the Hornets able to celebrate after guaranteeing another season in the second tier.