Searching and searching informations about this issue I found this very clear explanation:
“It’s important to understand what happened here. Vegas is extremely smart if you don’t lie to it. Setting the deinterlace to none is NEVER a good idea unless you are working with 100% progressive footage. If ANY of your footage is interlaced, you need to tell Vegas how you would like it to be deinterlaced should it ever need to. Vegas will take care of the fact that some of the footage is upper field first while others are lower field first. Like I said, it is very smart and knows what to do if you don’t stop it”
John Rofrano, Videographer VASST.
Everyone agrees with this?
Thank you very much Theo for your implication.