VOB editing with FFMPEG I read that it is better to extract from vob H264 files within a .ts to have a lossless and more accurate sample, easy to cut, is that correct ?

I used these commands :

ffmpeg -i VTS_01_1.VOB  -acodec libfaac -aq 100 -ac 2 -vol 2048 -vcodec libx264 -preset veryslow -crf 18 -threads 2 manu1.mp4

ffmpeg -i manu1.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts intermediate1.ts


I did extract two video samples from the intermediate .ts file with that kind of command : 

ffmpeg -ss 43 -i intermediate1.ts -vcodec copy -vframes 1400 -acodec copy 1-manubus.ts


if I play separately this .ts files in vlc, they are both completely decoded, no frames are missing

Finalization :

ffmpeg -analyzeduration 200M -probesize 150M -i "concat:1-manubus.ts|2-manuchoisit.ts" -c:v libx264 -crf 18 -preset slow -bsf:a aac_adtstoasc -force_key_frames 0 1990-Manu_Redpants.mp4 


But then, When I play the final file, the first 6 sec are a frozen frame and then comes the audio and video streams perfectly synchro until the end. If I play the first part 1-manubus.ts, the 6 first seconds exists and are not missing, what's happening ? Something related to first frame not being the I-frames ?
In the first step you are doing a lossy conversion, you transcode from vob to mp4, and then to ts. For a lossless re-mux you should just re-mux, better to specify both video and audio:

ffmpeg -i VTS_01_1.VOB -c:v copy -c:a copy -bsf:v h264_mp4toannexb -f mpegts intermediate1.ts


However, if you re-mux for the purpose of slicing then you should be aware that with this method it is possible only on exact keyframe boundary, so you might want to reconsider whether you want to do it this way. For a detailed explanation please see in this answer.