My take on the events following Luke's decision to fall is that Vader did use the Force to guide Luke's fall into one of the exhaust pipes. There was no obvious reason why Luke's body suddenly veered into the exhaust pipe. I didn't see a large exhaust fan or tractor beam pulling Luke in. It's reasonable to presume Vader was acting from a distance as he watched Luke plummet.
But why Vader didn't immediately levitate Luke when he stepped off the gantry? There are three possible reasons:
1) Vader was caught off guard by Luke's sudden decision to commit suicide.
2) Given Vader was likely exhausted from the duel, and in addition seriously injured by one of Luke's blows, he may have been unable to levitate Luke with Force powers in his weakened state (although we might surmise he had just enough strength for a gentle Force "push" into the exhaust pipe). Certainly Luke was too weak and scared (and unskilled with using the Force) to levitate himself off the gantry to safety, so some degree of concentration is required to lift heavy objects.
3) Vader wanted Luke to willingly join the Dark Side - or die:
There is no escape. You must join me or die. Don't make me
destroy you here... The Emperor is strong with The Force. But if
you join me, together we could overthrow Him. Do not resist. It
is our destiny!
Although this answer would contradict Vader's actions in saving Luke from falling to his death if Vader Force pushed Luke into the exhaust tube. Unless Vader was bluffing with the ultimatum "Join me or die."