Powerpoint has a feature to dump out the entire slide show as a series of JPEG images in one shot. Make sure you use a high enough resolution to match your video format, as Powerpoint may default to small images like 320x240.

In Vegas you can browse to the directory containing the images, click the last one, then Shift-click the first, and drag to your timeline. This imports all of them in order in one shot. They'll all be 5 seconds long, or whatever default still image duration you have set in Options / Editing. You can then adjust the timing of each individual slide on the timeline if necessary.

What you'll lose is any animations or timing you have set up in your presentation. You'll just get still images of the final appearance of each slide and all the same duration. If you do have animation and critical timings then you may indeed wish to do a screen capture into a video file instead.

To export Powerpoint at higher resolution, search the web for a workaround that allows you to export higher quality jpg images. For example, here.

Make changes to your registry at your own risk. But it does work :)

In PowerPoint, choose Save As and select the image format you want (I usually use png). PowerPoint will ask if you want to save the current slide or all of them. If you save all of them, PowerPoint will create a folder based on the file name you specify and create an image for each slide. I do this almost every day -- works like a charm.

There is also an option, at least in PowerPoint 2010, to export a slideshow as a .wmv file.

Thanks everyone for the advice.
I'm going to try the WMV export option first, since it will export in 1920x1080 raster.

Keith