Gareth Barry and Jlloyd Samuel combined on the left flank to punish some hesitancy by Ian Pearce and when Samuel crossed to the near post, Vassell nipped in ahead of Christian Dailly to net his eighth Premiership goal of the season with his left foot from six yards.
A draw would have been the fairest result between two teams who created few clear cut scoring chances in a scrappy affair in which possession was conceded too easily and regularly by both sides.
West Ham managed just two shots on target - a left foot volley by Jermain Defoe that went straight at Peter Schmeichel and a close range effort from Joe Cole, who could only fire at the giant Dane after finding space at the near post.
That was the best chance Hammers created after Paolo Di Canio put them ahead from the penalty spot after 13 minutes. Defoe was scythed down by Olof Mellberg, leaving referee Graham Barber no option but to point to the spot.
Di Canio, returning after a three-match suspension, didn't try to burst the net as he sidefooted his spot kick low, just right of centre as Schmeichel dived to the Italian's left.
The West Ham skipper also managed Hammers' only other goal attempt of a scrappy first half when he tested Schmeichel with a 30-yard screamer.
By then, Villa were back on level terms thanks to a 25th minute header from Juan Pablo Angel. He met Gareth Barry's curling cross from the left, although David James will probably be disappointed that he could only parry Angel's header into the net.
The England keeper deserved credit, though, when he dived full length to save a stinging long-ranger from Thomas Hitzlsperger.
James reminded his former club of his talents again in the first minute of the second half when he again dived at full stretch to tip over a rasping Barry volley and was again tested midway through the half by Hitzlsperger.
Tactically, Glenn Roeder selected a back three of Tomas Repka flanked by Pearce and Dailly, but wing-backs Sebastien Schemmel and Nigel Winterburn rarely ventured over the halfway line and were effectively part of a five-man defence that looked occasionally unsettled by the pace and trickery of match-winner Vassell.
Roeder switched to 4-4-2 when he brought on Vladimir Labant to play wide on the left in place of Richard Garcia and the visitors enjoyed slightly more possession, if no end product.
Joe Cole, Trevor Sinclair and Garcia formed a central midfield trio until the youngster departed but too often Defoe and Di Canio were isolated up front and lacking support from midfield, until Cole finally got forward for his great chance 14 minutes from time. With no threat from either flank and the midfield making little impact, Defoe had no service apart from a Di Canio through ball that led to his second half volley.
With Don Hutchison and Steve Lomas ruled out by knee injuries following last week's home win over Middlesbrough, they could ill afford to lose the match-winner from that game, Fredi Kanoute, who awoke with a heavy cold on the morning of the match and was sent home.
Minute-by-minute:
1 - West Ham produce a lively start and Di Canio's left wing cross is almost met by Garcia.
2 - Vassell pounces on a loose ball and fires a shot just wide from the edge of the box.
10 - Angel releases Barry on the left, but Repka does well to block the cross.
12 - Hammers are presented with a fantastic chance to open the scoring as Defoe, looking isolated and with no route to goal seemingly available, collects a rebound from his own shot and wriggles into the area before being brought down by a clumsy challenge from Mellberg. Referee Barber takes his time before pointing to the spot.
13 - GOAL! Paolo Di Canio coolly slots home the penalty to give Hammers a deserved lead. Aston Villa 0 West Ham United 1.
14 - From Staunton's inswinging free-kick, Angel heads wide of the far post.
21 - A Villa corner bounces around the Hammers penalty area before Barry flicks on to Vassell, who heads just inches wide of the post.
23 - GOAL.Villa grab an equaliser that, in all honesty, was on the cards. Again it is a dangerous cross from the left that causes problems for the Hammers, as Barry's centre is met by the head of Angel, whose effort is almost saved by James but drops into the bottom corner. Aston Villa 1 West Ham United 1.
26 - Pearce's header only reaches Hadji, whose volley from the edge of the area is deflected over for a corner.
30 - After excellent hold-up play from Defoe, Di Canio's blistering 25-yard shot is arrowing towards the top corner before Schmeichel superbly tips the ball around the post for a corner.
31 - Hitzlsperger follows up Di Canio's effort with a scorcher of his own from 30 yards out that James parries away.
43 - After Dailly's foul on Hitzlsperger, Staunton's direct free-kick sails high over the bar.
Added time: 1 minute.
45 - From Samuel's deep cross, Angel's bullet header is well held by James at his near post.
Half-time: Aston Villa 1 West Ham United 1.
Second Half:
46 - A long throw reaches Angel, who touches the ball back for Barry to unleash a stinging volley that James magnificently claws round the post at full stretch.
54 - Pearce booked for foul on Angel.
55 - Staunton's free-kick is gathered by Barry, who chests down in the six-yard box before a last ditch tackle from Dailly clears the danger.
68 - Substitution for West Ham, Labant replaces Garcia.
70 - Di Canio's looped pass over the top of the defence sets Defoe away, but the youngster blasts his left-footed shot straight at Schmeichel.
73 - Another fierce long range effort from Hitzlsperger is again superbly kept out by the outstretched hand of James.
77 - A great chance for the Hammers to take the lead, as Labant's curled cross is excellently controlled by Cole, whose left-foot shot from six yards out is somehow kept out by the massive frame of Schmeichel.
78 - Substitution for Aston Villa, Merson replaces Hadji.
90 - Vassell volleys high over the bar from Angel's flick-on.
Added time: 2 minutes.
90 - GOAL. Disaster strikes as, with almost the last kick of the game, Villa snatch victory. Samuel beats Pearce to the ball and crosses low into the six yard box, where Vassell is waiting to guide the ball past James. Aston Villa 2 West Ham United 1.
Full-time: Aston Villa 2 West Ham United 1
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