Darth Vader sets a trap for Luke at Bespin by capturing his friends and torturing them. The torture triggers a Force vision in Luke. Lucky for him, Yoda was nearby to tell him it was the future he was seeing. Based on this intel, Luke flies off to save them, falling into the trap.
If Yoda had not been there, would Luke have recognized the vision as the future? It seems doubtful. He may have discounted it as a dream/nightmare. Even if the vision were recurring, it would still take a conceivably long time for Luke to realize (without assistance) that he needed to do something. Ergo, without Yoda's help interpreting the vision, he could not have been expected to fall for the trap in any reasonable time frame.
Add to that the idea that an untrained Force user might not be able to receive more than an impression as we see Leia have on occasion.
Given that Vader did not know about Yoda, he knew that Obi-wan only had a matter of hours to train Luke before dying. How then, was he so certain that by creating the situation that might trigger a Force vision that only a trained Jedi could correctly interpret, that Luke would get the message and come to Bespin?