I find his obsession with designer brands and expensive cars to be pure vanity.

vanity 是名词，来自形容词 vain（虚荣的、自负的），指"过度关注外表、财富或地位的行为或态度"，即"虚荣心"。这个词带有贬义色彩，暗示这种追求是肤浅而无意义的。