I resolved my problem and learned something new about Vegas in the process. When I was on my shoot, I switched from progressive to interlaced on the same tape, the captured without scene detection, so each tape was one large file. When I looked at the properties for each of my captured files, they register as either progressive or interlaced. Unfortunately one of the files was both but, since it has to be labeled as one or the other, Vegas had called that one Progressive (I presume because it began with progressive footage). So when the render hit interlaced footage from that source file, it mishandled it, treating it as Progressive footage.
solution: I rendered that entire source file out to an interlaced file and, from within my edit, I replaced the “progressive” file with the interlaced file (pointed it to the new file).
lesson learned: don’t switch between progressive and interlaced on the same tape unless you plan to capture with scene detection enabled.