Como dizer "homens mais vaidosos que as mulheres" em inglês

"Man are more vain than women".

Corrigindo:"Men are more vain than women"

My suggestion:

men are vainer than women = As mulheres (são) mais vaidosas [do] que os homens

I- Para "Long Adjectves"
more + adjectivo + than
e.g.:
more beautiful than
more intelligent than
more boring than
more stubborn than

II- Para os "Short Adjectives"
1-adjectivos curtos que terminam em "e" + r
e.g.:
nice = simpático: (nicer than = mais simpático do que)
large = grande: (larger than = maior que)

2- adjectivos curtos + er
tall = alto: (taller than = mai alto do que)
vain = vaidoso: (vainer than = mais vaidoso do que)

3- para os adjectivos que terminam em "y", troca-se o "y" por "ier"
easy = fácil: (easier than = mais fácil do que)
busy = ocupado, atarefado: (busier than = mais fácil [do] que)

Complemento:

''The reversal of roles - I was just wondering, does anyone else find it strange that men today care more about their appearance than women do?''

Atenção para o uso de "vain":

''If someone said you are vain, what does that mean? ... It means that they believe that you care too much about your appearance.'' Help.com

What does "You're so vain" mean? - Yahoo! Answers

''Vain'' pretty much means that you love yourself, or that you think really highly of yourself. "3. Excessively proud of one's appearance or accomplishments; conceited.''

"What does vain mean?''

"It means you care too much about how you look.''