The forceful New Zealander, who spent a year at Bradford when he first came to Great Britain, claimed two tries in four minutes near the end to halt the Vikings' run of five games without a win and inflict a defeat on his former side which puts a further dent in their play-off hopes.
The Bulls, who have now won just one match in their last eight, led 18-14, then 28-26 but three tries in 10 minutes, including Ah Van's double just after the hour, swung the momentum decisively in favour of the Cheshire side.
Ah Van's first score was a spectacular 80-metre break from deep in his own half while his second needed a marvellous finish, diving in at the corner as two Bulls defenders tried to barge him into touch.
"Some of the tries he's scored, he's up there with some of the best wingers in the world," claimed a delighted Betts as he savoured the Vikings' first victory at Odsal since September 1990.
"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' opposite number Francis Cummins was left to rue the Bulls' failure to hammer home their first half advantage. Tries from Adam Sidlow, Keith Lulia and James Donaldson against a lone score from Rhys Hanbury enabled Bradford to carve out an 18-4 advantage.
But Stefan Marsh scored another vital touchdown for Widnes just 14 seconds before the break, then Hanbury claimed his second try within three minutes of the restart and the out-of-form Bulls always seemed under pressure thereafter.
Elliot Kear and Michael Platt tries opened the gap up to 12 points again but Marsh's second score, around the hour mark, and Ah Van's quick-fire party pieces, plus a total of four out of six conversions from winger Jack Owens, earned Widnes the victory.
Cummins said: "To go from being so dominant to throwing the game away in the second half, I can't describe how disappointed I am. We gave them nearly everything they got and good luck to them - they carried on and won the game."
- Sports & Recreation