The City striker, facing Villa for the first time since his £2million summer move to Manchester, scored twice in the opening half to help propel his side into fourth place in the Premiership.
But Villa's fourth defeat in their last five league matches leaves David O'Leary's side dangerously close to the foot of the table.
Villa could not have made a worse start as they lost goalkeeper Thomas Sorensen before the match had even kicked off after he injured his back in the warm-up.
Reserve Stuart Taylor, who was handed a surprise Premiership debut, made a nightmare start. A horrendous mix-up involving the keeper and left-back Jlloyd Samuel resulted in the left-back heading past his keeper who was racing from his line and that left Vassell with an open goal in only the fourth minute.
Vassell was denied a second goal by a brilliant save by Taylor who turned his shot onto the upright.
But there was no stopping Vassell, a Villa player from between the ages of ten and 25, and he was on target again in the 26th minute after a brilliant move involving Andrew Cole and ending with the former Villa man finding the net with an unstoppable volley, his fifth goal of the season.
City had only one moment of anxiety in the opening period when David James pulled off a brilliant save to turn away Steven Davis' goalbound shot.
The second half continued in a similar vein with City remaining on top and continuing to carve out chance after chance with Cole wasting one glorious goalscoring attempt.
Then, midway through the second period Villa were given an unexpected lifeline when central defender Liam Ridgewell was left unmarked at the far post to stab home a cross from Patrik Berger.
Suddenly City lost their impetus and had to endure some anxious moments before Cole sealed victory in the 83rd minute.
This time Vassell repaid the compliment for Cole whose shot took a wicked deflection off Mark Delaney before looping over the hapless Taylor.