In the Empire Strikes Back, how was Vader able to ambush Luke in the control room when he goes back after being flown out of the window, without Luke hearing his breathing sound? Vader just sort of popped up out of nowhere. Wouldn't Luke be able to hear his breathing sound, giving away his location?
It is established, in Shadows of the Empire, that Darth Vader is capable of overriding his breathing apparatus, though at the cost of a not-insignificant amount of pain. There is no reason he could not have simply held his breath until Luke was nearby.
As an alternative, any room like that is going to have quite the background hum from machinery. If Vader kept his breathing shallow enough, it could well be lost in the background gurgle.
Lord Vader was holding his breath. Exterior canon is unnecessary; watch the scene in question, paying special attention to the audio queues:
It's quite windy in the air shaft, but the noise becomes manageable once inside the hallway. There, it is quiet enough to hear Luke's footsteps, who ducks - in between Vader inhaling at last, and igniting his lightsaber.
It is this sharp intake of breath that does give Vader away. His breathing continues to be uncharacteristically labored until he's provided a respite, once Luke is laying on the floor "beaten". We finally hear Vader's breathing return to its normal ominous cadence while he says to Luke, "Don't let yourself be destroyed as Obi-Wan did."
From the Wookieepedia page on Darth Vader's Armor:
The ventilator was implanted in his terribly scarred chest, along with tubes that ran directly into his damaged lungs, and others that entered his burned throat, so that should the chest plate or belt control panels develop a glitch, he could breathe unassisted air for a limited time out of his mouth, which had not been burned to the extent of his throat.
A belt-mounted function box housed the respiratory sensor matrix responsible for controlling Vader's breathing. Using it, Vader could easily adjust the controls on his armor to increase or decrease his supply of oxygen.
Vader's breathing was machine-regulated, but his pace of breathing quickened during physical exertion and could be overridden by involuntary emotional reactions. In addition, his respirator would also let out a strained rasp if undergoing extreme exertion rendering him exhausted or if part of his armor is significantly damaged. This is especially evident when Vader ended up having his right arm cut off by Luke Skywalker in their final duel. The respirator slowed down or sped up in response to changes in his remaining body's functions, but Vader was unable to stop it or hold his breath, nor could he slow it. Vader's breather functioned even while Vader was speaking. However, it is apparent that a dampener or similar device had been installed into his suit by 3 ABY, as he was able to surprise Luke Skywalker in a dark passage near the carbon-freezing chamber on Bespin. Vader's ambush was only possible because he could temporarily quiet his loud breathing.
The script is quite clear on the quiet of the room:
INT. CLOUD CITY - CARBON-FREEZING CHAMBER
Luke rises into the chamber, borne by the platform. The room is deathly quiet. Very little steam escapes the pipes and no one else seems to be in the large room. Warily, Luke walks toward the stairway.
Steam begins to build up in the chamber. Looking up through the steam, Luke sees a dark figure standing on a walkway above him.
Jedi knights are able to cloak their presence, they are also able to just feel other force users nearby.
Keep in mind how both of them feel the presence of the other on separate vessels during the movies.
How it's possible then that Luke becomes ambushed? Easy... because Vader is more powerful in the force and he can cloak her presence from Luke.
Force users tend to left their "usual" senses behind (you can see the training with Yoda, how Luke is taught to ignore what he sees, and focus in what he feels), on the scene we are discussing Luke is on enemy territory, so he's probably trying to feel the presence of his father, and ignoring the information from his other senses.