The length of the samples will, until certain point, determine how metallic the VB sounds. The metallic sound of the rendering is in part caused by the way UTAU makes long notes, which is by simply stretching the samples, instead of looping a determined region of it. This way, extremely short samples (like defoko's) will sound very metallic, and even quite distorted, compared to longer samples. Of course, VERY long samples are useless and space consuming, mainly cause it is virtually impossible to have long samples with a stable volume so we could use ALL of it, so we need to cut them. And ideal length of samples is between 1 and 2 seconds, not too shorter, not too longer than this.