So all three have valid arguments @_@

Vital Strike over Blood Bath (for the speed build only)

Blood Bath is a damn good skill, IF you can reliably survive more than 3 round it would be better, but I don't think that a speed slayer without heavy armor (that shit slows you down without any Armor Movement skill) will survive that long. Vital Strike is not only used for an Agi debuff, it is a generally huge debuff for every opponent.

Examples are always for late game Blood Gods opponents:
After a successful Vital Strike the opponent get a ~120 Stat debuff depending on the location you hit, that's a huge debuff. 120 Str or Agi is roughly 4 pieces of orange armor, you basically strip some of his armor away for whole 3 turns!! Int does work a bit different and I wouldn't recommend to aim for that debuff (maybe vs trixster for science).
Rage glads are always a problem, you could prolong the fight with a Str debuff and weaken most of their skill or use the Agi debuff to slow them down. (should work best vs dodge rage glads)
It doesn't look that strong in the lower brackets, even in primus the debuff is only -32 which is laughably low.
Imagine 120 Str as encumbrance, should be 1-1,5% overall, it won't affect everyone but some glads are barely on the 25% encumbrance this could mess with the weight quite a bit.
That's the only justification I have for Vital Strike over Blood Bath.


Vanish on a slayer:

Yes it triggers rarely with a Whirlwind/Strike style and a bit less than reliable with our most defensive styles Lunge/Slash/Bash. We have to see it on a lvl 55 fully geared glad to draw more conclusions. Just for now we have to belief (4 skills deep into trixster it better be worth it late game)


And something for Smallz to tinker with:

Slayer are the only class that can use slashing/bashing weapons together effectively. War glads could in theory use a sword and mace together, but they lack a common fighting style other than tank.
By effectively I mean without wasting precious skills, a rage with a sword/mace combo is possible, but that world be a true waste.
Slayer are the only class to suffer from to many skills that no one wants.
They have Whirlwind/Strike which works with all weapon types, one could use an axe and gladius without any downsides (besides stat points, you need equal Str and Agi for good damage). As many glads have already almost equal Str/Agi for consistency purposes, a slayer shouldn't be hindered by it.
This opens a unexplored segment of weapon usage which barely anyone has tested or is currently using. The only guy other than myself would be "Murakami" and he is only using a Swords/Exotic combo which is the most "normal" approach we can take. And yes, it is not a waste of skill points to have 2 or 3 weapon skills at once. (Mojito has 2 weapon skills and the "Weapons Master" skill and does just fine.)
(Brief note, I used gladius/hatchet, mace/claw, axe/whip on a weak blue geared slayer with an unfinished skilltree so my experience is also rather low here, but it worked. Both weapons did the expected damage and sometimes Bash was an option. MAYBE just MAYBE there is a hidden gem in form of a very unique slayer build that rules them all @_@)