Looking around for this topic, I only find semiclassical calculations for high spinning strings. I would expect to find some calculation where the vertex operator formalism is used to calculate the amplitudes and then the decay rate for a superstring state decaying into two and into three superstring states, both in the massless case and in the case where the original string have some mass greater than zero. I hoped to be able to find a comparison between the decay of a fermion state into boson plus fermion and the decay of a boson state intro two fermions. C'mon, it is tree-level. Has nobody, in 30 years of superstring theory, calculated this kind of examples?