Tsuna literally just played in VSaikyo while having a Niji member on her team, with Sakura Aoi in the same lobby- the zero interaction rule probably only applies to enemies of the state like Doki/Sayu/etc. Luckily it worked out for Vivi in this case as she has some experience with SF6, but it would've sucked if she was also at beginner level and was forced to compete in a higher bracket.
It could also be some of the streamer servers for GTA/Rust/etc, and conventions as well (although she seems to have been invited to a tonne of them regardless). Makes you wonder if Niji tried to pull some shit at Anime Expo, would be funny (but unlikely) if the real reason they cancelled the concert was just out of spite.