Quick Update on Vital Strike, the debuff is roughly 40% off the given stat. Just like Adrenalin Rush, Hamstring and other skills the % will slightly vary about 1-6% depending on your own stats.
So basically Vital Strike is a 40% debuff for Str, Agi or Int (I falsely assumed that Int would be different, but no all three stats are equally decreased. Most glads just rock only minimal Int)

Normal values for main stats are usually at least 200, therefor we can expect an ≥80 Stat debuff most of the time. (Int will be around ~50)

Bodyparts and effects:

Head = Int
Hands = Str
Torso = Str or Agi (either the higher stat is chosen or it is just a simple 50/50 chance)
Legs = Agi

(Maybe Vital Strike is also nicely suited for the Vanish Slayer, as our main damage source is Blood Bath and lowering the opponents stats could give us one or two more rounds of bleed damage. The only thing to test now is the Int debuff and his effects. Unexpected low Int values could be the most detrimental part for opponents.)