Vegas is ever-improving and frankly, the stuff Vegas does well, it does spooky well.
Vegas is much more system-fault tolerant than PPro is... The disks hiccup and don't serve up a frame as fast as the software wants, high bitrate formats that are a little bit beyond the system's capabilities, etc. In many of these cases, Vegas might keep running where PPro might crash.
Vegas has a bundle that would include Sony's DVD Architect and I believe they have a bundle that would also give you audio post capabilities. PPro comes with Encore for DVD/BluRay authoring, and OnLocation for DV, HDV, DVCProHD P2 video acquisition.
I haven't looked closely at Vegas 9, but I'd say PPro's title tool is a definite advantage for PPro. PPro's Media Encoder runs in the background, but you can open another instance of Vegas on the same machine... CS4's integration between PPro, Photoshop, After Effects, Soundbooth/Audition and Encore is simply head and shoulders above anything else in the marketplace. The selling point there is the speed gained by skipping all those render down times. If you needed to use Vegas to edit, you'd be moving to Photoshop to manipulate images, or AE to create an effect, then you'd have to create some sort of compatible output file (which would include a render in AE)...and then open Vegas an import it.