Maybe the only reason Vader can contact him there is because Yoda is dying.

The cave was meant to be strong in the darkside anyway (unless that was Yoda fibbing as part of the test) so perhaps Vader can only get to him there.

Either way it's not a major plot impedance because Yoda is dead a few minutes later and anything Vader could have gained from detecting him is lost.

You could even shift around lines from later in the movie, "The darkside is your master now" to imply that Vader knows Yoda is about to die and maybe even that he or the Emperor had a hand in his sudden illness and impending death.

It would be good to keep the ESB vibe going as long as possible before going into the action again.