Hi Peter!
Apologies for the inconvenience here. You are correct that we don’t currently support indexing and querying vectors that are nested. We are looking at adding this to our roadmap in the near future, but in the meantime you will have to have this field at the root level of your collection.
I will make a note to make it more clear in both the $search knnBeta documentation and $vectorSearch documentation that this isn’t supported today. Let me know if I can help in any other way!