The unusual egg tag is LAYS_UNUSUAL_EGGS:(item):(item subtype)
It wont read the 3rd part that defines which sort of vermin the game should use.
That is incorrect
- the syntax is [LAYS_UNUSUAL_EGGS:item:material], where "item" is an item+subtype (e.g. WEAPON:ITEM_WEAPON_WHIP, THRONE:NONE, or VERMIN:NONE) and "material" is whatever the object is made of (e.g. INORGANIC:IRON, CREATURE_MAT:CAT:LEATHER, or "CREATURE_NAME:NONE" if you happen to be creating a vermin).
I just modded chickens to use [LAYS_UNUSUAL_EGGS:VERMIN:NONE:SPIDER_CAVE:NONE], and they did indeed leave live cave spiders in their nest boxes, but once I deconstructed them, the spiders immediately ran away at hyper speed and didn't seem to leave any webs behind, so creating vermin as items presumably doesn't initialize all of their properties correctly.