If it's done without any attempt to comprehend why someone would make the choice, to interrogate your assumption that "vanity" is the only possible reason for it, then yes. It's a prejudicial evaluation rooted in your caricature of someone's nature.

Of course, it's an elective choice, and in that way very different from prejudice founded on appearance. But the incomplete analysis, and the contemptuous reduction of the target's motivations, are in common with racist tropes.