As I'm sure you know

, during the past several event weekends for pies/ciders/cupcakes/apples/eggs/bunnies etc... I've spent over 17 hours a day farming both Raptors & Vaettir via. Hundred Blades for raptors and Perma Sin for kegging. (and I can guarantee you -- taking leechers does not increase your droprate of loot-scaled items -_-)
Although THEORETICALLY you actually kill more raptors than vaettir, I actually got more event item drops doing Keg Farming.
To put it into perspective; at top speeds
It takes about 55s to do a raptor farm which includes;run to cave
gather aggro
wait for aggro to settle & kill (33 enemies)
loot
resign
load into town
run out, load into rata sum
Now with Vaettir, it took about (with a perfect build) 87s to do a full run, which again, includes;Load into Jaga
Aggro + Cast perma
Ball everything (36 enemies) - Balling all 60 at the same time, or the other 24 after, no matter what, increases average kill time per enemy, no matter how you do it.
Step + keg spam
Loot
Run to exit & load
Turn around and load back into Jaga
So they are both a full cycle, no skewed results here.
So in the same amount of time, you are killing approximately .600 raptors every second, and .414 vaettir ever second.
It equates out to killing 45 raptors, but only 31 vaettir in 75seconds.
In my "experiment" doing keg-bombing actually gave me more (how is this possible?) event items by about 1.5x (2stack / 2hr from raptors vs 3stacks / 2hr from kegging)
It works out to about every 8.5 raptors drop an event item, and every 4 vaettir drop an event item.
tl;dr:
Although raptors SHOULD drop more event items, vaettir drop more than twice as many. If you can farm 36 vaettir in about 1:45 or less, do it.