Not gonna ask about how it works w.r.t. a natural 20 or a crit as that seems to be changing again, unless it just did already.
But how does it work for the following:
- Off hand for dual wielding
- 2nd or more attacks with main hand in a round
- 2nd or more with the off hand
- A cleaved attack, or a great cleaved attack, if the previous attacks were not vorpal deaths
- Same, but the previous attack(s) were vorpal deaths
- Glancing damage (ya never know! )