Thanks again Jeremy.
Not to beat this thing anymore but just did a quick test. I created a timeline with 1080p30 sequence setting. Dropped in the Vcam clip and it was fine. Then dropped in an HDX clip (with field dominance changed from upper to none) and it wants to be rendered.
Conversely, using the same timeline setting (1080p30) I dropped in the HDX clip first and it asked me if I want to change the settings to match the clip. If I say yes the sequence setting changes to 1080i60.
Are there any other settings that need to be changed on the HDX clip other than field dominance?