For a long time, he did his best to hobble discussion about metallic hydrogen. It's unusable as rocket fuel, but it's a recent discovery (although it was unlikely before) and developers of KSP2 latched onto the idea, despite several people on the forum telling them it doesn't work. Vanamonde was complicit in spreading that myth. While I think the devs themselves were simply insulated from the feedback by the "community management" team, Vanamonde has seen all the arguments (what with being the head moderator) and chose to ignore them. He even deleted some of the posts himself. As such, I hold him responsible for putting his hype for a magic drive above actual science.
There's a few other times where he censored discussion, ostensibly to enforce the rules, when his favorite posters started losing arguments due to their disconnect from the facts. However, in those cases he didn't air the lies himself. I'm sure there's other stuff, but as his favorite method is to just delete posts in a rather brain-dead way (leaving only a condescending message of his own), I only know about what he did to me and a few other people he screwed over.