Loan watch: Assombalonga downs Port Vale
Vale, flying high in the League Two automatic places, were beaten at home by the Shrimpers with Assombalonga grabbing the vital second.
The result leaves the Essex side one place and four points outside of the play-off's with a game in hand.
In the Championship, Craig Forsyth got another 90 minutes under his belt as Derby County lost at Birmingham City.
The Rams went down 3-1 to the Blues, with Forsyth playing the entire match.
In League One, Carl Dickinson's Coventry City were winners in Steven Pressley's first match in charge.
Dickinson played the full 90 minutes as the Sky Blues overcame Scunthorpe United 2-1 at Glandford Park.
In League Two, midfielder Ross Jenkins made his debut for Barnet.
The Bees were 4-1 victors over Morecame at Underhill, with Jenkins playing 90 minutes for Edgar Davids' men.
Steve Leo Beleck came off the bench for Gary Smith's Stevenage with 31 minutes remaining but was unable to prevent 2-0 defeat to Preston North End.
In the Blue Square South, Piero Mingoia came on for the final 25 minutes of Boreham Wood's 0-0 draw at Dorchester Town.
Stephen McGinn (Shrewsbury Town), Chris Iwelumo (Oldham Ahtletic) and Lee Hodson (Brentford) were all unused substitutes for their respective sides.