No, in Spanish we do not employ "rapidez" in the same way as English word speed is used with the sense you mention in the question.
I'm a physicist and I would use the term "velocidad" either for the vector or for the modulus (or, as you call it, magnitude) of the vector: in case of possible ambiguity, I would say "el módulo de la velocidad", but in many contexts I would produce sentences such as "la velocidad del móvil es de 25 m/s" without any problem.
Another example: the "speed of light" is called in physics "la velocidad de la luz".
Having a look at Google Books, I've found some physics texts that use "celeridad" to refer to the modulus of the velocity vector. Personally, I have never used such word with that meaning and I've not seen it in the Spanish texts I've used (either as a student or in my professional life). Many other physicists I known do not use it either.
In common life, we also use "velocidad" for the modulus of the vector (average or instantaneous velocity). We do not use "rapidez" to express such concept.
"Rapidez" is mainly used to express the quality of being fast or acting quickly. For example, here is a sentence from the book Yo he sido casada by Rafael López de Haro,
Le agradecí a Susana la rapidez con que vino en mi ayuda
that I would more or less translate as "I thanked Susana for coming to help me so quickly".