I love soaking up the ambiance in Arkham city. As an eavesdropper, I usually listen in on every henchman dialogue I can find. Vicki Vale occasionally broadcasts on Gotham FM which you can listen to using the cryptographic sequencer. She can also be heard from a radio in the church when you rescue the medical personnel from the henchmen. During the course of the game, when can I tune in to listen to her updates regarding Arkham city, using the sequencer or another radio?
No particular time, unfortunately.
Outside of the timers on certain objectives, there's just simply no concept of time in Arkham City. Vicki Vale and other broadcasts are just like the random banter of henchmen -- no connection to anything, really, just more-or-less random ambiance to deepen your immersion. (Ironically, that realization destroyed my immersion...)
If you stay tuned in long enough, and especially if you don't advance the main plot, you will start to hear good ol' Vicki repeat herself; conversely, tune in right after accomplishing major objectives, and you'll often (but not always) hear her talking about your most recent exploits.