Thank you for posting a MediaInfo report:

Frame rate mode             : Variable

That could be the problem - Vegas Pro often doesn't work well with Variable Frame Rate (VFR) video. When transcoding the phone footage, select Constant Frame Rate (CFR) if your transcoder has that option. Otherwise use another transcoder such HandBrake (free) - https://handbrake.fr/ - and see if transcoding to CFR fixes the problem in Vegas Pro.

Thanks, I will try with HandBrake

Well, so I did not use HandBrake yet, seems that changing the project frame rate to Double Pal (59.940) made the renderer to work. Now I just noticed that during the render there was written Good(Full) on the preview instead of the common Best (Full) and the resulting file is quite big (almost 10 GB for a 80 minutes video), too. Is there anything wrong with my setting? I use Voukoder and the previously done and rendered project is much smaller file (almost 4 GB for a 90 minutes video).

Oh, I noticed the rendering setting has Good instead of Best now, not sure why, as I never had it like that before... perhaps I clicked accidentaly.

And I just want to add (in case somebody search for possible solutions to similar problem), I often struggle to make VEGAS render, in this version 20 the working preferences configuration for my computer seems to be the following:

In the Video Tab: Dynamic RAM Preview - 0

Maximum Rendering Threads - 1

GPU Acceleration: Optimal (Nvidia RTX 2600)

File IO: Enable both legacy AVC HEVC, Raw processor: CUDA

Deprecated Features: Allow legacy GPU

That's disabling a decade worth of features including GPU encoding and decoding. Did you test the changes one at a time to isolate which ones matter?

Yes, I spent hours testing all possible combinations (crashes and freeze renders at different points), and in my case, similar problem occured also in previous versions (I use the software since 2020, so 17, 18 - that one I actually did not really make ever working well -, 19 - that one worked much better -, and now 20), though it depends on the situation (file formats, effects, plugins, etc.). For example, in VEGAS Pro 17 I needed to disable that so4compound something dll in the internal tab (I understand the legacy render does this, too?), etc.

According to what I read from some other posts, I am perhaps not the only user who didn't manage to reap many benefits from this decade worth of features. Still hoping there will be once a VEGAS version working out of the box for me (speaking of rendering now, because the timeline in 20 runs amazingly, the best so far!).

Interestingly, that file I managed to render from our phone is still not all right, does not really play correctly from the start, and it brings some error messages when I tried to drag it in or convert in both VideoReDo and VirtualDub 2, so I am going back to the first step and will try the HandBrake conversion to some CFR format.

All right, thanks everybody, I just tried to convert a short render excerpt (one minute video, the same setting like before and still the same problem in the resulting file) using HandBrake to CFR (same framerate like the render), and it now plays smoothly, seeking through the file also no problem (and the video looks and sounds OK to me, too, I kept the same quality setting - 17). I will try with the full video now and report back. Looking good...

If you run it through Handbrake try reverting all the other changes to defaults and they just might work well for you with massive speed improvements over using the legacy decoders and encoders.

I will try this, too (I expect that should make render faster, though I need to add the converting time - I should mention that this problem happens also when using the M2TS video file from my Panasonic camcoder, that one was solved by converting it in VideoReDo - Handbrake should do the similar job, can it join the M2TS files?). And after I converted that bad phone render fully, the result is great, file not too big and I cannot see any difference in the quality.

Anyway, except for rendering speed, everything runs smooth and fast, and (most importantly) stable in VEGAS Pro 20 for me with current settings, so I cannot complain. I did not even need to make any video proxy this time.