I have the same 30p option for my Canon camera and when I start a Vegas project by selecting “match properties” it fills in the options as if it was a 60i source (with de-interlacing selected). I have rendered projects both with de-interlacing selected and rendered as a progressive source and find little difference in the result (convincing myself that the progressive render is slightly better). I, too, would feel much more confident if Vegas would recognize the source file as a progressive source.
Does anyone have more technical detail to help us solve the “30p wrapped in a 60i wrapper” question?