Hi David,
I agree with your assessment that it is an interlacing issue. In particular, it sounds like a field order problem. As you probably know, interlaced video is captured as two fields which are temporally separated in time (either upper field first or lower field first). If you play these back in the wrong order, you will get the problem you are seeing.
This is complicated by the fact that HD is Upper Field First (UFF) and DV is Lower Field First (LFF) and you have a mix in your project. The first thing to check is the properties of the clips to make sure that Vegas has correctly interpreted the footage and has the correct filed order. One quick fix might be to reverse the field order in the render. So if you are rendering UFF switch it to LFF and see if that fixes the problem.
Somewhere in there your field order is getting misinterpreted and you need to find it and change it.
~jr
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