Fett could radio the Empire about what planet they're approaching and still follow the Falcon in. Lando said that "they" arrived not "they all" arrived right before Han and company, which could just mean "the Empire" arrived first; "the Empire" would make sense grammatically as the antecedent for "they" since Lando was just talking about the deal he'd just made that (he believed) would keep "the Empire" out forever. 3PO only saw stormtroopers before he was shot.With Fett. A tracking device....or keying into the one the Empire left on it on the Death Star. If Solo managed to do anything besides head for Bespin, Fett would know about it pretty quickly.
Given that Lando said that "they" arrived shortly before Han did, I would guess Fett tracked Solo to the edge of the Bespin system, than he and Vader jumped to the planet and "negotiated" with Calrissian.
By this point, the tracker on the Falcon is gone, or else Vader wouldn't have needed all those probe droids.