I've been reading too much fanfic, and now I can't remember:  who has actually dusted a vamp by twisting its head off?

Not twisting, but Buffy dusts one by slamming a car door really hard on it's neck till it's dust I think in "All the Way."

Angel might have in S1 of AtS. His trademark was the neck break that season.

I thought Spike did this in the Episode where Dawn was necking with a guy who turned out to be a vampire.  Also, Adam twisted a vamp's head off.