A couple of days ago, I too was very confused that the low C of the viola had a vibrato. Even more puzzling is the fact that to me it actually sounds like a "real" vibrato and not like an "interpolated" vibrato, as JWL calls it (i.e., a vibrato on a neighbouring, resonant string).
I strongly second stevesong's opinion that the lowest string should have a non-vibrato sound as a default. The standard, default way of playing the lowest note of a string instrument shouldn't be only available to the owners of the extended library.
I strongly second stevesong's opinion that the lowest string should have a non-vibrato sound as a default. The standard, default way of playing the lowest note of a string instrument shouldn't be only available to the owners of the extended library.