There were four changes in the Swans starting line-up with Richard Duffy and Kevin Austin coming into defence, Tom Williams in midfield and Darryl Duffy up front.
Port Vale included former Cardiff City players Rhys Weston and Malvin Kamara.
Trundle beat the offside trap in the 11th minute but, with only Joe Anyon in the Vale goal to beat, he rolled his shot well wide of the far post.
Swansea's leading marksman finally broke the deadlock with his 17th goal of the season in the 19th minute. Vale failed to clear a long throw into the danger area and the unmarked Trundle was waiting to pick up the loose ball and rifle it into the net.
They doubled their lead seven minutes later with a delightful move involving Dennis Lawrence. He started it in his own half before laying the ball into the path of Darryl Duffy who had the easy task of side-footing into the corner of the net from close range.
The Swans had to defend at the start of the second half but a superb run from Leon Britton led to the third goal in the 59th minute.
He slipped a pass down the centre to Darryl Duffy who jinked past Anyon before sliding in his second of the afternoon.
Roberto Martinez used all three substitutes and both Pawel Abbott and Tom Butler missed goal-scoring opportunities but the match was already won to leave the Swans well and truly in the play-off race.