Ok, I think I understand the problem of the angles of the bottom of the Cloud City. It is all about camera angle. In the top shot, the Falcon is traveling perpendicular to the core of the city, which is on the left as seen from the cockpit.
The shot of Luke hanging from the vane and the Falcon approaching is filmed as from the center of the city, or with the center of the city behind the camera. Because of this, the city doesn't slope down from the left or the right, but appears to be sloping up farther in the distance.
I made up one of my rare mock-ups do demonstrate the camera angles. Granted, I don't think the placement of the models (both the TIE's and the Falcon) is consistent in all of the shots, but I think the sloping of the city is correct and a slope doesn't need to be added to the shots on the vane.
Thanks to savmagoett for the stills. And the top mock-up has Luke and the Falcon in each other's spots, but it is the camera angles that I am concerned with.