Yah, it would be nice if the powerbooks and cinema displays could play without everything being stretched slightly. (most noticeable are the radars)

Try setting the widescreen resolution manually in the config.ini file. You can type in the amount of pixels for the height and width. It may not work for now but it's worth a try. In the future I'm sure they'd allow you to manually set it in a way. I know UT2K3 and Q3A allow it via ini manipulation. Why not Vendetta?

Vendetta enumerates all available resolutions from the OS, so if a resolution isn't displayed that means it isn't available. The stretching problem may be a result of the acpect of the screen not matching the selected resolution. For example, on a standard 17" monitor, 1280x1024 causes the stuff to be stretched whereas 1280x960 is fine.
So a 20" cinema display should be fine if you use 1680x1050 (or some res with the same aspect: 1900x1200, 1280x800, 1024x640, 800x500). You can try typing those into the config.ini file and if they don't work Vendetta tries to use a supported resolution.

Hmmm... the HUD doesn't scale up or down with the resolution, correct? In other words it's locked at the same size for all resolutions. Doesn't that mean that with widescreen displays it'll always stretch out? I mean the graphics for the HUD uses only one aspect ratio, no? Or does it compensate for that?

It should compensate for the different aspects.