Barrie McKay's second-half strike was enough for Fleetwood to condemn struggling Doncaster to a ninth defeat from 12 matches.
Fleetwood enjoyed the better of a lacklustre first half with good chances to break the deadlock.
Sam Finley's dink over Doncaster keeper Louis Jones forced player-manager Andy Butler to head off the line.
Vassell cut out a slack pass and surged forward before striking the post with a shot from the edge of the box.
Fleetwood took the lead on 51 minutes when Swansea loanee McKay raced in from the left and finished well from a tight angle.
Vassell went close to immediately doubling the advantage when he looked to catch Jones out from a long ball.
The closest Doncaster came to an equaliser was with a Reece James shot that hit the side netting.
Callum Camps should have doubled Fleetwood's lead but fired wide from close range while Ged Garner drew a good save from Jones as the visitors saw out the win comfortably.