Plouton, direct drop: approximately 2/120
Vagary campaign (x1 items): 0/39
Vagary campaign (x2 items): 1/28 [dropped in 1st slot]
Mars orbs: 0/299
Venus orbs: 0/162
Don't know who wrote KI turn-in has better drop rates
during campaign on BGWiki, but it honestly feels like a 2% chance per slot like the direct drop, just 2 chances instead of usual 1. And for those of you who are thinking of storing the other 31 drops in your bag to force rerolls, we are only seeing about a 33% chance for gear, so you will still see a lot of material and memory/codex drops.
If purchasing via Trove mercs isn't an option, I would honestly suggest doing direct if farming for a single person and campaign if you need multiples for a group.