Theo(@3POINT) and I discovered this yesterday.
When you have no interlaced presets made, in the same projectsettings with the same interlaced file on the timeline will this give always a progressive renderresult choosing “Make Movie/Save it to your harddrive/ Mp4>>>Next.

With one self made preset present in C:\Users\username\AppData\Roaming\VEGAS\Render Templates  that output is interlaced after exactly the same choises made.

My remaining questions are
- Is this made by design?

- If yess, do I miss something in manual or helpfiles about this?

PS.
I see the same behaviour in VMS PL15.
Not in VMS PL 14, because there it is for me impossible to render through Make Movie a interlaced mp4 file in an interlace project. Everytime the program freezes although I made a preset for the Sony AVC. Rendering that way a progessive file in a progressive project gives no problem.