My take: if it’s official promotional material, it’s not a spoiler in the classic sense. If trailers were to count as spoilers, we’d have:

The Official Rogue One Spoiler Thread
The Official Rogue One Non-Spoiler Thread
The Offical Rogue One Trailer Thread

…the last one being for people who want to discuss the official trailer without being exposed to real spoilers and leaks, while not revealing trailer content to people who don’t want to see it.

That would be silly.

If you have a gripe with the trailer having Vader in it, then your gripe is with the studio, not with people who want to discuss the movie and official material related to it.