The vibraphone has a relatively "cool" and "pure" bell-like sound. It doesn't have that vocal warmth of a saxophone. So what it takes to warm it up, to compensate for this, is the use of vibrato. That's what makes it a "vibra"phone and not just a marimba or xylophone. What Milt Jackson does is to slow down this vibrato, creating a soulful oscillation that counterbalances the harmonic purity of the timbre. Miles Davis comes at the trumpet from the opposite direction, reducing vibrato to a minimum, so that the absence of oscillation itself gains a unique emotional power. It's not just the absence of vibrato, but the tension created by this absence. Coolness is not absence of heat but heat contained. Pitch is itself a rhythm. A vibrato then is the oscillation of a pitch, the rapid variation between two pitches close to each other. It can be narrow or wide, slow or fast, or varied. Think of how some horn players' vibratos become faster or wider toward the end of a long note.