So to start off, let’s list how much crit severity each version of vorpal gives and add it to our base crit severity (75%):
None: 0% + 75% = 75%
Lesser: 12% + 75% = 87%
Normal: 25% + 75% = 100%
Greater: 38% + 75% = 113%
Perfect: 50% + 75% = 125%
This means that your crits will do 75% more (than non-crits) with no vorpal, 87% more damage with a lesser, 100% more with a normal, etc..
Let’s say you did a non-crit hit of 10,000 damage. If that hit did crit, it would produce the following damage:
None: 10,000 x 1.75 = 17,500
Lesser: 10,000 x 1.87 = 18,700
Normal: 10,000 x 2.00 = 20,000
Greater: 10,000 x 2.13 = 21,300
Perfect: 10,000 x 2.25 = 22,500
Now most people seem to think that since a perfect vorpal increases your crit severity by 50%, it will increase your overall damage by 50% x Your Crit Chance. So, let’s do the math.
22,500 – 17,500 = 5000 <- Perfect vorpal does 5000 more damage than no vorpal
5000 / 17,500 = 0.2857 x 100 = about 28.6% more damage
So your crits will deal 28.6% more damage with a perfect vorpal compared to no vorpal.
Now multiply this by your crit chance. To make it simple, we’ll say you have 50% crit chance (most PvE rogues go for):
28.6 x 0.5 = 14.3%
This means that a perfect vorpal will increase your overall damage by about 14.3% (assuming you have a 50% chance to crit).
Note: These calculations don’t take into account crit severity increasing feats (such as the phantasmal destruction feat in the Renegade path for CWs)