I, too once thought as Bill Wyld did!!
And it IS an excellent answer, but I would like to, if I may - posit another - which I think is a little bit more fun, and JUST fits with only one tiny and quite possible retcon
Hope you enjoy this.
VADER'S FLEET HAS A L R E A D Y MADE A HYPERSPACE JUMP IN AN ATTEMPT TO "pursue" THE FALCON ALONG IT'S MOST LOGICAL FLIGHT PATH WITH THE
FALCON "PIGGY-BACKING" NEATLY ON THE BACK OF THE STAR DESTROYER'S COMMAND TOWER!!!!!
There is another way out of it, it's not perfect but it's feasible. When Han & Leia are going through the Falcon's System Directory, they specify that they are in the 'Noad' (Anoat?) system. As we know it is impossible to get inter-system travel at anything less than moving up into Hyperspace, because it would take centuries (you can't have entire solar systems within sub-light travelling distance of each other - the systems would just collapse gravitationally towards each other)
My proposition is, even though it appears to contradict the straight forward flow of events, is that the imperials have ALREADY made one hyperspace jump in an attempt to pursue the Millennium Falcon in the direction it was headed when they lost track of it. Because the falcon is tucked neatly behind the main tower of one of the Star Destroyer's* they are not in any danger of being"ripped" off the main vessel as the command tower protects then from any forces that would do that, assuming even that there are such forces in "hyperspace", or more to the point, accelerating to enter hyperspace.
In short, the Millennium Falcon is effectively "part" of the Star Destroyer it is magnetically latched on to for the purposes of that jump.
(*Star Destroyers - the empire are SO heavy handed ~ if they're not blowing up planets they're destroying "stars" apparently)
Having arrived at the first best "guess" point - probably taking into account how much fuel and the capacities of a craft like the Falcon - OR maybe this is simply the next safe Hyperspace "drop out" point - the Imperials scan the area, but can't find any trace of the craft. Cue the conversation about "every" drop out point in this direction to be calculated and visited. The reason the Imperials still have garbage to dump is because they made this "pursuit" jump in HASTE- it was a long shot and it didn't work. Thus Vader's insistence to sort of "keep this up".
So - We're near the Noad (Anoat?) system, having just piggy backed a short jump though Hyperspace care of the Empire. Bespin is a PLANET in the Noad (Anoat?)system, where Noad (Anoat?) is the name of the Sun of that system and not the planet. If I remember rightly Bespin is identified as the name of the planet not the system, even if it is referenced as such, we can put that down to people using lazy short hand. After all, wouldn't we refer to our Sol System as Earth's System*, because it's the planet with inhabitation**?
*(*see Bill Wyld's excellent answer above where he establishes a similar proposition)
(** I understand that BESPIN almost certainly does not have an indigenous population, as it really does appear to be a Gas Giant, but the Mining Operation is legit, and would be known about. So anyone travelling to the Noad/Anoat system would be talking about the only place to hang out as readily as the name of the sun. "You know - what's it called. There's a big mining city there. Er - the Bespin system. In Anoat.")*
So there you go. Bespin is a planet in the Noad (Anoat?) system, which the Empire has just unknowingly given their prey a free lift too. Dropping out of hyperspace just outside the Noad (Anoat?) system - which (Notwithstanding FORCE AWAKENS somewhat alarming emergence within the atmosphere of "Star Killer base"scene) is HOW YOU're SUPPOSED TO DO IT (!) - and the Falcon now only has to make the same sort of trip as they did to the rebel outpost on Yavin in SW4:"A New Hope". More or less. And they can spend months or weeks getting to know Lando, or be that bit further out from the Noad (Anoat?) system that it takes them however long you want to believe Luke needs to complete his training - the beauty of this idea is that the time frame is flexible depending on how far or near you think Vader's Fleet emerged from the System with a Sun called "Noad" (Anoat?), with Bespin as the "Jupiter" of that system.
I understand that there might be fiction and SW encyclopedia stuff that could contradict this proposition, but it is honestly the least painful retcon I can see.
While there is nothing to prove, or even indicate this unseen jump in a Bespin-y - direction, there is nothing to contradict it either - also the two "Vader Fleet" (+ Han/Leia cockpit scene) scenes are broken up by a Luke/Yoda scene, and that is how I was able to propose it - that little Hyperspace jump fits in neatly while we're away catching up with events on Dagobah. I mean - wouldn't you pursue your prey immediately if you were the Empire in that situation? And the disappointment of that long shot not working out leads to the conversations between Vader and crew. Admittedly, when we return after the Dagobah cutaway, poor old Captain of Vader's flagship is just falling to his knees care of Force Grip, but maybe that's because they've just made their pursuit jump (and his life was hanging in the balance, dependent on this pursuit jump yielding up the Falcon?)
Yeah, it's a retcon, and it's not perfect, but I want to submit it for consideration. The most important reason for putting this forward is that in this slightly flaky, "non-science" universe of Lucas-Space, the Story is telling us that all that chasing the Millennium Falcon around the place has gotten everybody FAR from their starting point, so far that we are now all nearer another system altogether, and Lando's place is now within Sub-light travel distance. That's essentially what the Narrative is telling us. All I've done is add one tiny possible explanation as to how this might be physically possible - because logically, so far, the Falcon has fled from Hoth, with Imperial Battleships in pursuit, into an Asteroid Field - and then DOUBLE BACKED and Headed Right Towards them the way they've all come to play Blink With Them!! So without this retcon - they're still knocking about around not far from the asteroid belt in the Hoth System! Have at me fellow Star Wars Cannonites!
May the Force Be With You.