To paraphrase Eric's perfect answer,

Leave it alone. If the source reports correctly either lower or upper, Vegas will get it right 100%. They are mixable on the same timeline.

If there is line twitter, it is wrong source media, and that is where you would experiment by changing it.

There are other things that can cause this too, all related to bad source.

You can only do whatever works. For me, nothing ever did unless I deinterlaced everything and treated it as progressive. You might have better luck.

TBH, a workflow for me often consists of a preliminary decomb in Handbrake, and exporting a near lossless progressive mp4 (RF 1) back into Vegas. Although it can be done door to door in Vegas, I like the results of OCD-ness a little better.

You need to bear in mind that not all files correctly report their field order. I assume Vegas just reads the file header and won't know that it's incorrect. Fortunately you are able to correct it.

Of interest I once read that many, or most, video editors don't do complex field swaps when converting the field order. They just move the fields up one and replace the bottom row with either a copy of the row above or black.

I'm aware that there maybe not correctly reported field order, but that not is the problem. I try to flag field order on LFF footage to UFF but no luck. The problem is only when render LFF to UFF. When render UFF footage to LFF no problem. I will try other codecs, but I think the problem is before that.

Sory my gramar. :)

Of interest I once read that many, or most, video editors don't do complex field swaps when converting the field order. They just move the fields up one and replace the bottom row with either a copy of the row above or black.

Actually I don't understand what exactly you mean, but I try to move up very little the picture in Event Pan/Crop and that do the trick. Amazing! 😅

Here example

https://drive.google.com/file/d/14fiIz4cTMXqeHwat1RnUwf9mK_f6whwm/view?usp=sharing
https://drive.google.com/file/d/1amGCk7_uW-r9b0hU5dZRgn99ICYV2Wmn/view?usp=sharing

I think when change something in the picture Vegas' deinterlacing kicks in do the work.

Max, I've tried to replicate your LFF to UFF problem on my VMS15. I don't have Pro but Pro and VMS behave similarly.

My only LFF material is PAL DV Widescreen LFF first. Picking shots with plenty of movement and rendering with the Main Concept Mpeg2 codec UFF I get smooth movement. No problem at all. The field order has obviously been converted correctly.

Do you have a short LFF clip you could upload somewhere for us to download and play with?

Raw footage

https://drive.google.com/open?id=1-mkG4ZRU6Ko3DcEbCCQitSbmwGriG3V0

Having confirmed Eric's results 100%, I'll try to provide a little more background, in the confidence that it will give you a few dependable checkpoints in your workflow, going forward.

Max, I have examined your raw footage down to field level, and it is correctly stored at 25i, LFF. That is also what your source MEDIA PROPERTIES (not project properties) report, so those should not be changed under any circumstance. That done, your timeline plays back as expected, witnessed by isolating the fields in UFF or LFF 50i projects, and taking notice of correct unilateral field (and frame) stepping in a 50p project, all entirely as expected. Breathe out.

If I deliberately set Source Media Properties to UFF, stutter or backstepping occurs on random fields in a 50p project, so again it should not be attempted.

That said, I suspect your sensibilities are being overtaken by a couple of things you are doing or leaving out.

-- You may be changing fields manually and continuing editing while a project is open. This should not affect your render, but it certainly can upset preview recognition of your field order, that is until your project has been saved, closed, and reopened. The song says, "Don't Change Horses in the Middle of a Stream!"

-- You may be paying attention to the appearance of combing, real or residual, in the Preview during editing and rendering. If you would like to get a rough idea of playback on a deinterlacing player, which Vegas is decidedly not, you can set your Preview to Draft (not recommended), or temporarily set your Project Properties to 25p, remembering to set them back to LFF or UFF, using Match Media Settings, an icon in the upper right of your Project Properties, and choosing a file that represents the majority of your project assets. This sets your project right with the world; it still does not turn Vegas' preview into a player; however, changing project properties above the horizontal rule does not *necessarily affect the render (this is a really reassuring thing to remember).

*Of course, if you are choosing a render that matches ridiculous or nonsense Project settings, then you are entirely on your own (it's out of radar range as far as anyone here can determine). That's why Sony and Magix thoughtfully gave us Match Media Settings, which occurs by default on the first opened project file in later Vegas versions, and every time you wish to rematch, following a brief "excursion."

Following these steps exactly, I was able to, in a couple of minutes, load a bunch of files with mixed field orders, including yours, to a fresh timeline, set my Project Properties as above, and render a perfectly ordered file that works on VLC, and I assume other players.

Overanalyzing for me is normal, especially when confronted midpath with seemingly incongruous appearances. The solution for me is to push through the interruption and render a test; remembering I am not "wasting film." 😉 This can be done lickety-split (that means quickly), and I can judge authentic playback on a media player, not the Vegas preview.

(With my little chromecast, I can watch my renders on my big screen while editing on a laptop sitting in grandma's rocker, and eliminate a lot of ??? either preemptively or down the road). Hope this helped, and thanks for providing your source; it looks like you were in a cool place.

Oh and Max, I am very pleased and reassured that you came in with a real problem, but without carrying a chip on your shoulder. It's refreshing in the face of an onslaught of mediocre social media attitude on what used to be a more functional user support and discussion forum. Just thanks.