A late goal by league leaders Carlton Town denied Valley a deserved point on the road.
It looked like Tyrell Hamilton's debut goal midway through the first half was enough to secure a point but Lamin Manneh's 84th-minute strike made it six wins in a row for the hosts putting them ten points clear at the top for Christmas.
Rugby played well in the first half but it was Carlton who took an early lead with Manneh latching onto a through ball to send a low first time past Marcus Bald into the corner.
Valley levelled with Hamilton pounced on a loose ball in the area after Geoffrey Lundoloki's cross from the left was half cleared.
Although Rugby felt they should have led after the opening period Carlton kicked into an extra gear after the break and restricted Valley to very little.
The winning goal was an early Christmas present for Carlton as Valley goalkeeper Bald misjudged an innocuous high ball into the box and despite the first effort being blocked, Manneh was on hand to fire home the rebound into the empty net.
