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 following his second-half foul on Greg Cunningham, who had to be stretchered off.
Vassell's second goal of the season ended a run of four straight away defeats and ensured Leicester leapfrogged the Tigers in the npower Championship table.
The former England forward met Richie Wellens' cross with a superb volley which flew past debutant Hull keeper Bradley Guzan.
The Foxes could have had more, 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.