Ah Van struck twice in less than three minutes near the end to give the Vikings victory in a game in which they had trailed 18-4 midway through the first half.
It was 13th time lucky as the Cheshire side had gained just a point – a 25-25 draw in 2005 – from their previous dozen visits to the West Yorkshire venue dating back to September 1990.
Rhys Hanbury and Stefan Marsh also claimed two tries apiece as Widnes wiped away the memories of a string of near misses to claim their second away win of the season.
Bradford, who have now lost four matches in succession and won only one of their last eight, must have thought they had their play-off hopes back on track when they still led 28-16 with barely 20 minutes to play.
But three Vikings scores in the next 10 minutes gave the Cheshire outfit their first victory in six games.
Bulls began well with prop Adam Sidlow charging diagonally through a tempting gap to open the scoring. Luke Gale, with the first of four conversions adding the extras.
Keith Lulia helped increase the home side’s lead to 12-0 after 16 minutes when he carried over a superb off-load from Danny Addy.
The tide briefly turned when a high shot on Dave Allen put the Vikings in the danger zone for the first time.
Livewire fullback Hanbury then scythed through the home defence to score, although Jack Owens surprisingly missed the conversion.
A Nick Scruton break quickly had the Bulls on the attack again and Gale’s unexpected change of direction enabled James Donaldson to cross for his first try of the season.
Substitute Phil Joseph, a former Bradford player, barged over the whitewash with his first touch of the ball but lost control amid some desperate, yet effective, efforts to prevent him from grounding the ball.
However, Marsh gained reward for plenty of Widnes pressure late in the half by taking Kevin Brown’s pass and going over just 14 seconds before the break, Owens this time kicking the goal to leave Bradford 18-10 ahead.
Within three minutes of the restart Gareth Hock put Hanbury in for his second try with Owens’ fine touchline conversion closing the gap to just two points.
Quick hands from Brett Kearney and Adrian Purtell enabled Elliot Kear to squeeze over in the corner for a Bulls try but Gale’s first miss of the afternoon left the hosts still only six points clear.
But two minutes later Michael Platt broke down the other flank and a break and magnificent dummy left him free to race behind the posts to give Bradford much-needed breathing space.
Still the Vikings refused to lie down and Hanbury created a second try for Marsh just before the hour mark, although again Owens could not conquer the blustery wind and angle.
Ah Van then came to the fore, first collecting a Gale last tackle kick deep in his own half and racing 80 metres for the game’s most spectacular score, with Owen’s third conversion narrowing the deficit to two points again.
Barely three minutes later and Ah Van was in again at the corner and Owens completed the comeback with his fourth goal from six attempts from the touchline.
Bradford coach Francis Cummins was left to rue the Bulls’ failure to hammer home their first half advantage. “To go from being so dominant to throwing the game away in the second half, I can’t describe how disappointed I am,” he said.
“We gave them nearly everything they got and good luck to them – they carried on and won the game.
“We are edgy at the moment and we tried coming up with big plays to kill the game off whereas building pressure with different kicks would probably have been better.”
Widnes coach Denis Betts heaped praise on Ah Van. “Some of the tries he’s scored, he’s up there with some of the best wingers in the world,” he said.
“He’s a great guy to have in the team – the other boys love him and with his quality of finishing, he’s always likely to score tries.
“He has his socks rolled down around his ankles and looks half the time not to be bothered but he knows how to finish.”
Betts has lamented a run of narrow defeats this season but pointed to that as a partial reason for the fine comeback. He said: “We’ve shown character the last three or four weeks but it’s probably stood us in good stead – losing so many close games in recent weeks.
“They had to respond and showed lots of character and the right work ethic.
“I’m pleased for this group because they’ve worked really hard this week and, on the back of a draw and two losses, it would have been easy for them to have felt sorry for themselves and dropped their heads.
“We can see the light at the end of the tunnel and we’ve moved forward as a group in the last few weeks.”
Bradford: Kearney, Kear, Purtell, Lulia, Platt, Addy, Gale, Scruton, Diskin, Sidlow, Bateman, Blythe, Donaldson. Substitutes: L’Estrange, Manuokafoa, Olbison, Evans.
Widnes: Hanbury, Owens, Marsh, Phelps, Ah Van, K. Brown, Mellor, Gerrard, Clarke, O’Carroll, Hock, Allen, Cahill. Substitutes: Joseph, Kavanagh, White, Leuluai. Referee: R Hicks (RFL).