It’s a limitation of the AVI format, not Vegas. There’s no such thing as a native 24p AVI. Vegas can edit a 24p Quicktime perfectly natively. But it’s very nice to have the option to edit the captured files in either 60i or 24p.
There's no such thing as a DV compressed 24p AVI. That much is true. Avi is just a container format like Quicktime so you can compress using any format you want - including progressive formats w/ 23.9 or 24fps.