In the python implementation, the Binary class has a from_vector static method. You have certainly considered this solution, of having only a factory method instead of a new class. But the new methods toArray and getVectorType are specific to this binary type, so having a dedicated class makes sense. But if you create a new Binary($raw, Binary::TYPE_VECTOR) instance with the vector type from the raw binary data, you don't have this methods. The current state doesn't allow creating a BinaryVector from raw data. I think you would have to wrap in into a BSON Document and encode/decode it to get a BinaryVector instance. I don't find any other binary type that would require a specific class.

I see 3 possible implementations:

1. Add Binary::fromVector(array $vector, VectorType $vectorType) and toArray() to the existing Binary class
2. Create BinaryVector class that extends Binary (current state of this PR).
3. Create BinaryVector class that implements BinaryInterface, \JsonSerializable, Type, \Stringable.

We need to compare pro/con of each solution.

LGTM. No objection to adding corpus tests in a follow-up PR.