Which manga from Rosario + Vampier do I have to join after watching the Netflix Anime?
The part with the mirror of Lilith at the end of season 2 ("Rosario to Vampire Capu2") is at the beginning of "Rosario + Vampire Volume 10".
However, since some things are left out, shortened or presented differently in the anime, I would recommend you to read the manga from the beginning.
In volume 1 because the anime consist almost exclusively of fillers and hardly implement anything from the manga and if not then really well.