For historical reasons (Atlas Search implementation preceeded Drivers specification), a BSON binary element with subtype "Vector" (9) may be zero-width, containing none of the bytes specified by the BSON Binary Vector specification. Critically, this omission includes the two vector metadata bytes which the spec currently mandates as always-present.

For backward compatibility, given the server does not reject zero-width binary elements, client libraries should be permitted to allow such zero-length binary data as well. Given how niche this edge-case is, "MAY" is probably sufficient; "SHOULD" or "MUST" is likely unnecessary and may be deferred until real-world customer demands motivate Driver-wide alignment.