Learn Italian: Il problema è che stai diventando vanitoso. - The problem is that you are becoming vain.

To replace the adjective vanitoso, you use the invariable pronoun lo. You can place it before the conjugated verb stare (Non lo sto diventando) or attach it to the end of the gerund (Non sto diventandolo).

Could I just say diventi instead of stai diventando here?

Yes, but stai diventando emphasizes the ongoing, active process of becoming vain right now. While Italian uses the continuous tense (stare + gerund) less often than English does, it is very natural with verbs of gradual change like diventare (to become) or peggiorare (to worsen).

Quick refresher: why is it Il problema if the word ends in -a?

Problema is a masculine noun despite the -a ending. It belongs to a group of words of Greek origin that follow this pattern, which also includes everyday words like il sistema (the system), il programma (the program), and il clima (the climate).

Can I use the word vano instead of vanitoso to mean "vain"?