The 30-year-old striker, tempted back to English football by Sven-Goran Eriksson in November after a spell with Turkish side Ankaragucu, crashed in a fine volley as the Foxes won only their third match on the road this term.
Hull endured a miserable afternoon which was compounded by a red card for defender Liam Rosenior.
Vassell's second goal of the season ended a run of four straight away defeats and ensured Leicester leapfrogged the Tigers in the Championship table.
The former England forward had already got an early sight of goal when he fired wide in only the third minute.
But Vassell made no mistake eight minutes later when he met Richie Wellens' cross with a superb volley which flew past debutant Hull keeper Bradley Guzan.
The Foxes could have had more before the break, with Paul Gallagher firing a good chance wide and Lloyd Dyer getting round the back only to snatch at his shot when he actually had more time.
Cameron Stewart had Hull's best opportunity of the opening period with a shot which Chris Weale blocked, but the Tigers looked distinctly out of sorts.
Home boss Nigel Pearson threw on new signing Aaron Mclean and midfielder Jimmy Bullard in a bid to lift his side.
But Leicester still looked the more likely to find the net in the second half with Gallagher driving wide from 20 yards.
Hull's cause was not helped when Rosenior was shown a red card for a foul on Greg Cunningham just after the hour mark.
The tackle was late, rather than malicious, but teenage midfielder Cunningham came off worse and had to be given oxygen and stretchered off with a nasty-looking leg injury.
The Tigers did have the ball in the net through Paul McShane in the 72nd minute but the referee blew up for a foul on Weale.
Leicester could have had a second when Gallagher swung in a free-kick from the right but Vassell shot too close to Guzan.
But Hull offered little resistance and were deservedly condemned to their fourth home defeat of the season.