Vital spot - what is it?

Hey,

Iw been wondering what Vital spot attack really stands for.

Like in my case with 97 Othell. I was Fortune seeker before and wasnt able to Half kill mobs with Single dagger.

Afaik:
Base Vital rate is:
-10% from Maximum critical
- 5% from single dagger mastery

For other than fortune seeker the active skills increase vital (GH in this case):
- Mortal strike lvl8 -> 28% increased vital spot (has cd)
- Critical Prowess lvl1 -> 5% more vital spot


The thing is - that my GH does half-kills almost at same rate than my Fortune seeker. I cant see any visible difference in killing mobs in Blazing swamp at level97.

Also - an ex-hero dagger told me that atm Vital spot increases his damage only... basically his saying that Vital spot is not related to Half-kills.

Basically im just confused.
a) I cant HK any mobs almost at all in Blazing swamp with any Othell version. (this is with lvl97 Othell)
b) I dont understand what Vital spot attack rate stands for.


If anyone with solid knowledge could answer any of my questions I would hugely appreciate.


Best
RealMajor


Edit. Only thing I can think is - is that Vital Spot is actually chance to Half Kill indeed - but:
for case (a) the 28% I get from lvl7 Mortal strike is "increased" in a sense that it multiplies the total Vital spot bby 1.28. Thus if you have 20% base vital spot you get addition 5.6% from Mortal strike -> making the total Vital spot attack rate 25.6%...
For case (b) the only thing I can think is that since the Blazing swamp mobs are 98-99 Vital spot has very low "hit" rate. Even so low that any 97 Othell cant HK mobs there properly. I was told that some mobs in Dimensional Warp dont take HK at all. ( I have actually been told by a 99 dagger that he kills mobs there with few hits so its very possible that its possible that 97 Othell really cant grind effectively in Swamps.)