Vassell was in the right place at the right time at Princes Park in Kent to tuck away a cross from another substitute Les Afful.
And the win, which lifted City two places up the table to 14th, extended their unbeaten run to four matches since that 6-0 Bank Holiday Monday hammering at home to Oxford City.
He said: "It was our most complete performance of the season. All over the pitch we were good and could have won by more, but Ricey made a great save at the death to ensure we got the points."
With Rob Farkins and Pierce Mitchell unavailable, there was a recall for Jake Ash who had a great game in front of the back four.
That was the only change from the side that drew 2-2 at Weston-super-Mare in the week, though Truro suffered an early blow when Cody Cooke had to go off with a groin problem to be replaced by Ryan Brett who went onto have an excellent match.
After a relatively quiet opening 45 minutes, with City looking comfortable, the visitors started to create more after the break.
Man of the match Matt Wright, who went close in the first half, headed against the bar and had another cleared off the line.
Truro though were not be denied a famous win as they continue to come to terms with life at a higher level after winning promotion last season.
Truro City: Rice; White, Richards, Pugh, Riley-Lowe; Ash; Green, Cooke (Brett 21), Dawson, Wright (Afful 83), Duff (Vassell 64) . Sub (not used): Yeoman.
Goal: Vassell (86) 0-1.
Man of the match for Truro Matt Wright
Referee: Matthew Buonassisi.
(by Rhod Mitchell, West Briton)