Considering that your target would lose ~18 evasion from the AGI down (that's a whole 9% hit rate for you and everyone hitting the mob!), as well as upping the crit rate a bit? I think you should throw that before any 4-hit crit-based WS, yes.
I don't know about the casting time & MP cost, but in my book, debuffing enemies is often the best thing you can do when you stop thinking about your own epeen/performance and instead care about the party's kill speed.
I absolutely love debuffs. And trust me when I say that as a blu who solos quite a lot, the benefit of debuffing a mob is most certainly not lost on me. :P
I mostly asked in this case to see if it would ever be worth it before vorpal blade in particular. There are steps you can take beforehand to help yourself out, of course, I had just never seen anyone mention this spell for lowering a mob's AGI in order to help the critical rate. I was curious if it was possible, and how beneficial it could really be.
This spell is not usually very practical, otherwise. It only takes 1 set point, which is nice, but it has no beneficial stat effects (only a negative, INT-1), costs 37mp, takes 4 seconds to use, decays, is AOE magical, and only lasts 30 seconds or so. The frost damage from it IS 18/tic, but that only amounts to around 180 damage.
So, considering something like, say, mandibular bite is only one more mp, .5 sec cast time, 2 set points, single target physical, and GIVES one INT instead of take... it would take some figuring out.
Would the help to crit rate (if there is one) and the potential dmg from the frost effect - and other factors - outweigh the damage gained simply through casting another spell? Seems like so much to consider, really... at least, as far as I can figure. ;)