Photoshop itself isn't a viable format, is it? It's like trying to open an Acid file in Vegas-- you cannot, unless you render it to a different type.
Virtually all programs such as Photoshop allow user definable settings for size. JPEG's and other compressed formats are often scaleable to fit your needs...
But I'm confused as to the nature of your question-- are you using stills in a Vegas project? If you are, (and if that's your question) I suppose that it doesn't matter what still type you use-- your Vegas render should still be the same.
Really? Jeez-- this whole time I've been converting images to JPEG's before incorporating them...
Still though, I'm a little confused as to your question. Are you asking if there will be a final render-size difference in your Vegas files if you used another format?
The answer is no. If you put in .jpg og .png og .bmp or .gif - whatever, it should make no difference to the size of your rendered Vegas project.
If your files are getting too big you need to look into the 'render as' settings.
PSD's work well because you can use a single file as a source and destination. PSD's are widely supported in their native state, just watch out with adjustment layers and advanced features like that. they don't work out well in some programs. PNG is my file format of choice. PNG is the same as PSD in the way that you can use Fireworks to generate your graphics with an object oriented system and use the same PNG file in Vegas as a bitmap. it's pretty amazing.
I'd steer clear of jpegs since they just look crappier than a PNG or PSD and they can't handle any sort of transparency. the compression in a jpg is no good to Vegas. it has do decompress the image when it is layed onto the timeline and store it in RAM in an uncompressed state no matter what file format you use. if you are using jpgs from a digital camera, then it is best to just leave them as jpgs... there is no reason to convert them to PNG's or PSD's.
final render size is not affected by the format of the stills.
Stands for Portable Network Graphics. A file format designed for web graphics. It supports 24-bit colour with lossless compression, one alpha channel, and alpha transparency.