League leaders Daventry came out on top after a tense local derby at Butlin Road on Boxing Day.
A crowd of 625 saw Valley's unbeaten home record this season come to an end, with second half goals from Luke Emery and Adam Creaney securing the visitors what may prove to be a pivotal victory.
Town once more finished the game a player short, after Dominic Perkins received a second yellow not long after Daventry's opening goal - with Rugby boss Carl Adams also subsequently sent to the stands for dissent before the game's conclusion.
Richard Blythe returned to Adams's starting line-up having missed the previous match due to work commitments, with Fazel Koriya also restored in an unfamiliar full-back role in two changes from Town's 10-man win at Wellingborough.
It was the Northamptonshire side that made the most assured start though, with Taylor Orosz curling one wide after he had cut in from the left and Emery drawing a fine reaction save from Matt Hill when he helped on Kieran Fitzgerald's strike.
Rugby's first real chance of the game came when Blythe's looping header to Lewis Rankin's corner was cleared from the line by Creaney.
Adam Confue headed one wide of Hill's goal, before Valley found their feet around the half hour mark and finally started to dominate possession and territory.
Their best opportunity of the game soon followed, when Danico Johnson picked out an unmarked Rankin with a deft cross from the left, but Rankin dragged his effort agonisingly wide of Lustin Cerga's far post.
Cerga was then down well to keep out a further shot from Rankin, but the final chance of the half saw Town captain Loyiso Recci deflect Orosz's powerful strike over.
Emery screwed one wide in the new period's opening minutes, with Charlie Evans flashing a dangerous ball across the face of the goal at the other end.
The defining moment of the game came midway through the half, when Keenah Rosser clipped the heels of Emery in the box, and Creaney converted from the spot for the twelfth time this season.
Koriya did have a shot blocked as Rugby looked to bounce straight back, before the magnitude of that task was greatly escalated when Perkins was sent-off following a mass fracas near the dug-outs after Johnson had gone in late on Orosz.
The visitors comfortably saw out the game from there, and threatened a stretched Valley defence on the break on a number of occasions.
Orosz volleyed one just over and Emery had an on-target shot blocked by Koriya, before he was then given the freedom the Rugby penalty area before steering the ball past Hill for Daventry's second on 85 minutes.
Line-up: Hill; Rosser, Koriya, Recci, Francis; Hayden (Blake), Blythe, Perkins; Evans, Rankin, Johnson. Subs not used: Kavanagh, Holloway, Tulloch, Marsden. Booked: Perkins, Johnson. Sent Off: Perkins.
Daventry line-up: Cerga, Childs, Lukeman, Harris, Alexander, Confue, Fitzgerald, Cassidy, Emery, Creaney, Orosz. Subs: Anayiotis, Dumbleton, Henderson, Merrill, Finlay.