As Wikipedia says: "Tốc độ là độ lớn của vectơ vận tốc." I like the succinctness of Vietnamese applied to maths.
Yep. Speed is the magnitude of the velocity vector.
"Velocity is different from speed" would be more accurate. "The" is unnecessary & unnatural in both cases.
Unless you are talking about the velocity and the speed involved in a specified situation. Does Vietnamese make this distinction, I wonder.
'different from' = american english
'different to' = british english
But at the moment this doesn't accept 'different to'
'Velocity is different to speed' - not accepted. I think this is a perfectly valid and more succinct translation.