There is an NPC that is spawned via a settler buff station. This NPC will give you a buff that allows you to get these eggs as a drop from killing mobs in Grimvault. They are purely vendor trash and just an extra for of income. The NPC is easy to miss because it's not the usual big tube-like structure that you normally see at settler buff stations. I believe the NPC is labeled as a "Trophy Vendor" so it's easy to pass by not realizing she gives a buff. There are a few NPCs of this type that give buffs in a few different zone that do the same thing (just a different item that drops as opposed to these eggs). One of those other zones that comes to mind is Farside, where there is an epic item that drops from having a settler buff and those items are vendor trash just for extra money as well (so don't get too excited when you see lots of purples dropping in Farside, lol).