When playing back the entire timeline faster or slower, Vegas doesn't preserve pitch. Any - even minimal - speed up/down in Vegas playback speed mess with the voice pitch, bringing distortion (i.e. voices like Darth Vader or Mickey Mouse). For instance, in Adobe Premiere, by pressing L to speed up the playback, the pitch is much smoother than Vegas. If you press L in Vegas, Mickey Mouse is coming!
Vegas can already preserve pitch when you change the clip property "playback rate" individually. But emphasizing, the suggestion here is play the time line preserving pitch (e.g. changing the timeline playback rate in the bottom left control: http://i.imgur.com/JI3lSbk.png). Then, the great work Vegas already does with time stretched clips also become available when changing the playback rate of the timeline.
Many users need to speed up/down Vegas playback speed in their workflows (see links below for detailed use cases), but the distortion in voice pitch is a huge shortcoming currently.
If you need further info, please let me know.
Any change to have this feature requested included in Vegas roadmap?