I'm gonna go out on a limb here and suggest that your vectorLength variable is not a a power of 2
.
Try cropping your data such that its new length L = N^2, where integer N > 1.
Does that get ride of the exception?
Are you expecting your signal to be periodic in nature? Is it attached to some mechanism rotating/shaking at fairly steady frequency? If not, what would you hope to get out of using an FFT?