When I want to export a draft version in 720p of a 4k video, on the 720p I have a lot of elements that change places and stay in their position on the 4k version. So my preview is bad, I can't find a solution so that Vegas manages to put everything at the same resolution so that no matter the resolution, certain parts of my assembly do not change place
Not what you are asking, but why do you have a 50fps project and render it to 24fps? To avoid jerkiness try rendering to 50fps or 25fps instead (assuming the source media is 50fps).
Can you also share your render settings for 4K? If it's 4096 × 2160 that's a different aspect ratio than your 720p render and 1440p project (16:9).
If you're making a project to export as UHD or 4K I'd set the project resolution higher and then downsize at the render stage. Some VEGAS functions like track motion work at the project resolution, not the source resolution.
I rendered in 24fps because i was just looking for a way to quickly render the video and have a preview out of vegas for the current editing, yes it's a 1440p video, I didn't have a 4k example on hand, but both do the same thing, when I make a smaller render to get my colleagues' opinion quickly on what I did, it changes everything in terms of the positions of the images etc.
I believe the issue here is that the picture is being cropped because 720p captures a much smaller picture than 1,440p, if that makes any sense. There are some pictures at this link that show the photo range of a 720p video compared to a 4K one.
My recommendation would be to duplicate your project, save the duplicated project under another name, and set the project properties of the duplicate to 720p before rendering. I hope this helps!