It's a great question... if the current view is that unmeasured standard quantum particles don't have particular attributes for position, velocity, momentum, energy, etc , how much greater the problem of characterizing the string as having a vibration (which classically entails all those attributes)?
The size of a string compared to a proton diameter is about the same magnitude as the size of a man to the distance from here to Andromeda galaxy... the uncertainty principle would seem to make any possible "vibration" attribute of a string impossible.
I'm sure the string theorists are using the math to work with different kinds of abstract attributes altogether, and "vibration" is more like the nickname for an abstract mathematical attribute - sort of like "spin" is the nickname for intrinsic angular momentum of something not thought to be actually spinning...