Vassell, often a lone striker, produced his fourth goal in four games against Aston Villa this season as the Midlands club failed yet again to beat their bogey side.
The breakthrough occurred when Claudio Reyna found Vassell with a short pass and the City striker turned and shot left-footed into the corner of the net.
Villa surprisingly included three central defenders with the return of skipper Olof Mellberg. It was certainly experimental but against the lacklustre City attack it was not put under any real pressure until the goal poaching of Vassell.
There was a lack of real quality and the respective positions of the two teams was a fair indication of the standard of play on the night.
City had their early chances with Reyna going close with a header, while Vassell was guilty of some weak shooting from close range.
Villa raised the hopes of their fans when Mellberg closed in on a right-wing cross from Gareth Barry only for David James to foil his former colleague with a reflex save.
There is an obvious need for more quality in both teams and the respective managers are expected to busy during the close season boosting their squads to avoid a repetition of recent months.
Villa's defence was caught on the wrong foot by Albert Riera as he jinked his way through only for young Gary Cahill to make a timely intervention as the former Espanyol player was about to shoot.
Riera was foiled again early in the second half when his blistering shot was parried by Thomas Sorensen as City began to apply considerable pressure on the new-look Villa defence.
Most of the danger originated from Trevor Sinclair, but City struggled to capitalise upon the service from the former West Ham player.
Villa's improvement came after the introduction of Gabriel Agbonlahor for Milan Baros. Twice the highly rated young striker tested the capabilities of James with raking shots.
But it was Villa old boy, Vassell, who settled the issue with his smartly taken 71st minute strike as he demonstrated the frailties of the Villa defence.