So Vader and a garrison of troops under the help of Boba Fett land on Cloud City intent on capturing Han Solo and setting a trap for Luke Skywalker. Cloud City is essentially a peaceful mining facility with no real military or advanced police force. They have some firepower, but not enough to stave off a full-fledged attack. Vader and company know this and don't bring down a full invasion force to keep their presence hidden. Most likely, Vader and troops depart at their destination and then have Vader's shuttle (and any remaining ships - if they even brought more than the one) fly off to the other side of Cloud City or some other hidden area close by (not all the way back to the Executor (which may not even be at Cloud City, since Han and crew don't seem to detect its presence on arrival. The Executor could be some light years away awaiting a signal to return - who knows?)). And they wait.
The trap is sprung, Han is captured, Vader decides to keep Leia and Chewbacca, and Vader battles with Luke. At this time, everything is going exactly to plan. Most of the heroes are under wraps, the populace is unaware of the Empire's presence, and Vader has Luke exactly where he wants him. More ships/troops are still not needed. Eventually, Vader fails to turn/capture Luke and believes Luke to either be dead or at the very least unreachable. Time to leave, and still no need for more troops.
Here's where the troops are required: Having lost Luke, Vader needs Leia and Chewbacca more than ever. However, with the help of Lando, they escape their guard and alert the populace to the Empire's presence. The trap has failed and the city knows the Empire is there. There is no longer a need for subterfuge and Vader calls for his shuttle and apparently more TIE fighters.
As Vader awaits his shuttle and reinforcements, Lando and company are making their escape along with the rest of the city. Vader leaves the city, the Executor makes its reappearance from wherever it was and TIEs are launched to ensnare any escaping ships. One could assume their orders are to disable any ships and not destroy as they're after Leia and Chewbacca. Vader assumes if they are to escape somehow, they would certainly go for the Millennium Falcon (disabling of the hyperdrive and all) but can't be sure. He doesn't need some Imperial pilot blowing away his only link to Luke. The TIEs don't see the Falcon until it's already getting out of Dodge and then pursue.
Essentially, I think no TIEs should appear until the Falcon is already blasting out of Cloud City. There should be massive air traffic from the evacuation as the heroes try to make it to the Falcon, but no pursuing TIEs. From Vader's plan, there was no need for TIEs on hand in or around Cloud City in order to maintain the element of surprise. Once the plan falls to pieces, then the fighters are recalled. There simply wouldn't be enough time to have them flying around destroying ships (nor would Vader want them to do this) immediately after the Vader/Luke fight.
As the Falcon is escaping, that's when we could see a squadron of TIEs flying towards Cloud City and hence Lando's "What about those fighters?" line.