I want to start this post off by saying the shield Captain America was given is different than pure vibranium. His shield is a mix between proto-adamantium and vibranium. So if the descriptions are different, that's because they're not the same thing.
Dr. MacLain's experiments with proto-adamantium lead to the creation of true adamantium. This proto-adamantium (the only known source) was incorporated with the vibranium in the shield. The vibranium in the shield grants it unusual properties, allowing it to absorb virtually all of the kinetic impact from any blows that the shield receives without injuring Rogers in the process. The vibranium is also a factor in the way Rogers throws his shield: he often uses it to ricochet and strike multiple opponents or stationary objects with little loss of velocity in its forward movement after each impact.
This description purposfully leaves out the ability to absorb vibrations.
Now to pure vibranium.
Orginaly vibranium could only cut metal, well any metal known to man.
Vibranium first appeared in Daredevil #13 (February 1966), which was written by Stan Lee and illustrated by John Romita. Here, vibranium was seen to be an unusual metallic element with decidedly strange properties. Since that point in Marvel continuity, it has been established that there are a few variations of this element which can be found in isolated regions all around the world. The variation first introduced in Daredevil #13 eventually became known as Anti-Metal, with this variation's unique attribute being that it can cut through any metal known to man. .....
But later we are introduced the Wakandan variety:
Later in Fantastic Four #53 (August 1966), by Stan Lee and Jack Kirby, a newly debuted variation of vibranium was introduced in the isolated nation of Wakanda. This variation had the unique attribute of being strong textable to absorb sound. This is the variation which is most often identified in continuity as simply "vibranium".
Let's learn a little more about the Wakandan type, since this is the one you would like to know more about.
The Wakandan isotope possesses the ability to absorb all vibrations in the vicinity as well as kinetic energy directed at it. The energy absorbed is stored within the bonds between the molecules that make up the substance.**
So we know it absorbs vibrations in the vicinity, albeit we don't know the radius, and kinetic energy directed at it. The paragraph goes on to describe the kinetic energy absorption a little further.
There are limits to the capacity of the energy that can be stored, and although the exact limitations are not yet known, there have been a few examples. One such instance was when the oil conglomerate Roxxon discovered that a small island in the South Atlantic had a foundation composed of vibranium. Due to this, Roxxon found it necessary to destroy the island and so blew it up with bombs. Unable to absorb the force of the explosions, the vibranium was destroyed, but it did succeed in entirely absorbing the sound made by the explosion, preventing damage to the surrounding area.
So Wakandan vibranium absorbs all vibration in the area, including sound waves, and a limited amount of kinetic energy. So this should answer the question you have in your title.
But to clarify your confusion I'll follow with:
Since Captain America’s shield is made of a mixture, not pure vibranium, it seems like it may have lost the ability to absorb vibrations. Thus the description may be different and may only be able to absorb kinetic energy.