Hacken had the chance to ensure they would hold on to third no matter what would happen over the rest of the weekend, and move to within two points of Hammarby, but a dreadful eight-minute spell handed the victory to Vasteras, who are not out of the race for Europe themselves.
The away side went in front when the Hacken defence failed to deal with a cross from the left, and arriving into the area was Simon Gefvert, who caught the bouncing ball sweetly and sent it gliding into the bottom corner.
Four minutes later it was 2-0, as another cross from the left - this time a Mattias Hellisdal free-kick, was nodded in at the far post by Bonaventure Lendambi. Before the break, a dreadful spell for the hosts was compounded when Leo Vaisanen was sent off for denying Lendambi a clear goalscoring opportunity.
The free-kick Vasteras were awarded came to nothing, but when they had another right on the edge of the box with an hour gone, Hellisdal made it 3-0 by firing the ball straight into the top corner.
The 10 men had something to cheer late on when Christ Wawa finished well after being played through by Abdoulaye Doumbia, but it's two losses in four now for Hacken who will be hoping not to drop further than fourth this weekend. A first win in three takes Vasteras sixth, just two points shy of Getingarna.
With Sweden head coach Graham Potter watching on, Elfsborg moved into third - above Hacken on goal difference - and extended their run of form to four wins and a draw in their last five by beating Degerfors, who remain in the relegation play-off place after successive losses.
The deadlock was broken 13 minutes in, as the home side worked their way into the area with some intricate passing, before Julius Beck swung a cross to the back post, where Niklas Hult was waiting to turn it back across goal for Leo Ostman to stab into the net from close range.
1-0 was how things stayed until late on, though the lead was rarely threatened, as Degerfors generated just one shot on goal in 90 minutes. But the Boras side made sure nonetheless with 87 minutes on the clock, as Taylor Silverholt launched a counter-attack and played Ari Sigurpalsson through on goal, and he made no mistake with his finish.
Bottom-of-the-table Halmstad picked up just their second win all season thanks to a 92nd-minute winner against Kalmar, whose winless run in the league extended to five matches.
Having drawn with fellow strugglers Orgryte last time out, Halmstad looked to be heading for a second draw in a row, but things got even better in added time, when Ludvig Arvidsson picked up the ball in midfield and drove at the Kalmar defence, before taking aim from 25 yards and seeing his strike sail past the goalkeeper.
That goal ended a nine-game winless run for HBK, who are on a two-game winning run in all competitions thanks to a Svenska Cupen win in midweek. They stay bottom but move onto 11 points, eight shy of Kalmar in the 13th and final guaranteed safety position.