Actually Divx and Xvid are considered delivery formats. They generally make pretty lousy source video for editing projects. And H.264s are usually either MP4s or M2T AVCHD videos.
The ideal AVI codec, especially for older versions of the program like version 8, is the DV-AVI, the format created when you capture miniDV footage over a FireWire connection.
Being delivery formats doesn't prevent them from being source formats too. It can be said that AVC and MPEG are also delivery formats, but there are many cameras that record those formats.
I'm not sure i'd say that DV is an ideal format. It's limited color space handling, which is also different from so many other formats, makes it pretty cheesy. DivX at high bitrates can far outshine DV for detail and color fidelity. Personally, i'd say uncompressed AVI is more ideal than all other formats in all aspects except file size.
That kind of brings up another point. The difference between source formats and delivery formats isn't really so much the codec used as the amount of compression. Most any codec with minimal compression makes a dandy source, and most any codec with greater compression is well suited for delivery.
Each of those requires installing a separate vfw codec for the AVI wrapper.
Search the forums for some links.
None of those are included natively in Vegas, because they are all open source.