I know it isn't much data [and horribly sorted]...
But this data for killing lichens at various sizes was taken with all HUD spots empty (no vita/pickups of any kind). I didn't take enough data to even get 1 vitapod [in a total of 20 kills]. I suspect that the drops are additive for lichens when they combine, so the data would represent an estimated equivalent of 49 drops.

That being said, in the entire time of this thread, I lumped vitapods in the "item" category [along with vials and such]. Items constituted of about 36% of green boxes grabbing vitapods as soon as they appeared (from either monsters or boxes).

There have been no boxes that were empty. There was 1 that contained two items, and was not included with the dataset [despite being recorded up until that box].

---Raven Hypothesis below---
I suspect that the way treasure box drops function are that it rolls twice for the "base" drop and 1-9 times for the rarity [based on first roll].
The base drop would pick one of {crowns, heat, token, material, item, gear}. The second roll would be {# of crns, # of heat, type of token, type of material, type of item, type of gear}. My prediction would be that when you have a current vitapod, it would reroll either both rolls or just the second roll. I suspect it only rerolls the second roll.