This should be an enhancement, I can only select Bug as issue type.

The index name of $vectorSearch should only be required when multiple vector indexes are present. Mongo should throw an error if the index name is missing (or mis-spelled).

Ideally, the index name can be derived from $vectorSearch.path. If the index is unique for a path, then it should be used. This change is to improve user experience and provide some safety margin for usage.