Degradation. The key to vile is degradation, de-humanization, and the destruction of all that is bright: hope, faith, charity, love.
And all because YOU WANT IT.
And when the heroes/PCs offer you escape, you REFUSE.
Because this is what you enjoy, being an object (as in a table, not as in for companionship) or a victim.
Because you chose this.
Because this is the only life that fits.
NOT because you were enchanted or bespelled, but because this is what makes you feel good.
It's about choosing what is unhealthy, what is disgusting and strange, all the while knowing that.
Look at quite a bit of BDSM literature for inspiration here.
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It's about your worst sin saving your life.
Not by chance, as in time spent at the brothel saving you from the magical cataclysm.
No, not like that.
Unfortunately, I'm about to go to bed, so I'm low on off-the-cuff examples of this one
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It's about your virtues putting you in an untenable situation.
The exact opposite of the above, this means making virtue violate itself, or at least other virtues.
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The trouble is that vileness loses its oomph when it's compelled, and players can sniff that stuff coming. Subtlety is key.
Also NPCs who embody these points.
A vile world is one in which right, wrong, empowering, demeaning, are NOT opposites, but rather hazy dreamlands bordering on each other. The gore serves that.