Salford wound up a rollercoaster campaign with their fifth defeat in a row despite leading 4-0 at the break thanks to a Marc Sneyd try.
Widnes took charge after the interval to run in converted touchdowns from Ah Van (two) Rhys Hanbury and Joe Mellor, which means Brian Noble's men can still finish bottom if London Broncos pull off a shock win at Hull KR on Sunday.
Gareth Hock, one of 12 new signings announced by Salford on Thursday for 2014, made his farewell appearance for Widnes against his future employers.
It was also an emotional night for Lee Gaskell, Jodie Broughton and Jordan James who were among the Salford players on duty for the last time.
The Vikings handed 20-year-old centre Declan Hulme his debut and the Reds can have no complaints at the result with Kevin Brown outstanding.
Salford started the stronger and Darrell Griffin almost powered over after Lloyd White lost possession.
Sneyd pegged Widnes back with several neat kicks but the Reds struggled to break down a committed defence.
Salford had a let-off when Willie Isa spilled the ball and they took full advantage to take the lead five minutes before the break.
Broughton hacked on Ryan McGoldrick's kick and Sneyd won the race to touch down but was off target with the conversion attempt.
Hanbury did cross the line for Widnes but the try was disallowed by referee Tim Roby for a knock-on in the build-up.
Ah Van should have scored in the corner for the Vikings but fumbled a kick through to leave the Vikings trailing 4-0 at half-time.
But Widnes hit back straight from the restart after Mellor's 40-20 put Salford on the back foot.
Hanbury danced his way through a static defence and Jack Owens booted the conversion to edge the Vikings ahead 6-4.
A Brown kick brought an error from Danny Williams to gift Ah Van a try after 65 minutes, with Owens landed the goal to make it 12-4.
Then Mellor touched down for a controversial effort with Owens' kick stretching the advantage to 18-4.
Hock went close to crossing once more for Widnes before Ah Van exposed Salford's defensive frailties to score his second try in the last minute - with Hock booting the conversion.