"vaan" can stand for "vain" in spoken Finnish. The sentence decides its meaning.
Spoken form of 'vain': softens the wish
Viikonloppuna mä vaan lepään. - On the weekend I just rest.
vaan here: just - spoken form of 'vain'
Written Finnish: Viikonloppuna minä vain lepään.
Mä vaan katselen, kiitti. - I'm just looking, thanks.
vaan here: just - spoken 'vain'
Written Finnish: Minä vain katselen, kiitos.
Mee vaan, mä tuun perässä. - Go ahead, I'll come after you.
vaan here: just / go ahead - spoken 'vain'
Written Finnish: Mene vain, minä tulen perässä.
Se oli vaan jotenki hassu juttu. - It was just kind of a funny thing.
vaan here: just - spoken 'vain'
Written Finnish: Se oli vain jotenkin hassu juttu.
Hienoo, onnee vaan! - Great, congrats!
vaan here: just / simply - spoken 'vain', softens the wish
Written Finnish: Hienoa, onnea vain!
Mä vaan heitin läppää. - I was just joking around.
vaan here: just - spoken 'vain'
Written Finnish: Minä vain vitsailin.
Spoken and written Finnish are different registers. The examples show how "vaan" is used here and pair it with the written sentence. Learn the meaning in context rather than replacing every occurrence of a word.