Vector is a client for the Matrix communications platform (perhaps calling it a communications platform is wrong, it's more general than that).

Matrix itself is just a spec / API for sending and receiving messages. Tensor and Quaternion are two other clients that work with a given Matrix home-server. AFAIK all of these clients can utilize Matrix APIs to some degree, however I'm unsure about video conferencing as I only really use Vector. That said though, other clients' capabilities don't really depend on Vector at all, unless they're using https://vector.im as an identity server.