Everyone present in the hangar wanted that spacecraft to be yanked down by Vader. This includes the people who rigged it to take off.
The escape plan is contingent on Vader attempting to stop the first spacecraft, succeeding, and being distracted by tearing into the empty spacecraft. If Vader was not able to stop the spacecraft fleeing, he would have let it go, and possibly noticed the other spacecraft spinning up its engines in the hangar behind it, containing all the refugees and Kenobi.
As a result, the decoy spacecraft probably wasn't loaded with enough fuel to make a successful escape (the real escape spacecraft needs that more), and its engines may have been rigged to fail convincingly, allowing Vader to revel in his own power and mastery of the Force for the distracting moment when he realizes the other ship is empty.
Maybe Vader could have stopped the other, fully-loaded transport ship if he'd been able to bring his full powers to bear on the correct target at the time. At the very least, though, Kenobi and the refugees believe that he could have done something sufficiently decisive that it outweighed the benefits of having both spacecraft make a serious departure attempt, if both could. And Vader believes he has a chance of doing so, or he wouldn't have tried. But it's also possible that Vader isn't quite as strong as he initially appears in that scene.