Questions Vasquez Always Dies

I'm having a little difficulty with the Vasquez Always Dies trope. I get the general idea of 'the tougher more butch female characters dies while the weaker, more feminine female character lives' but then qualifies this with the idea of the more femenine woman being a secret agent type (and thus tougher and more competent than the Vasquez).

So I'm wondering is this trope based more on perception and character style or more on actual ability? For example if you have a tomboy character who swears a lot and brawls with others and a graceful, traditionally beautiful female character... who also happens to be a far more skilled fighter than the tomboy, then kill off the tomboy only, is the tomboy still a Vasquez?

Basically does the Vasquez actually have to be tougher and more capable or does she just have to fit a certain image and could plausibly be the less badass of the two?

No, I think it's pretty clearly an image thing. Having the more feminine character be indisputably the more competent one is just a case of Justified Trope.