Cassius Dio writes that, when Tiberius was preparing to depose Sejanus, Naevius Sutorius Macro was appointed to replace him as commander of the Praetorians, and worked with Graecinius Laco, who was the commander of the Vigiles to carry out his plans. (Dio, Roman History LVIII.9)
A lot of texts suggest however that it was Macro who was prefect of the Vigiles. Is Dio wrong? Is there a definitive source on Macro's function prior to 31 (inscriptional evidence perhaps)?
If Dio is right, was Macro merely an officer of the Praetorians or of the Vigiles? And if he is wrong, what was Laco's real function then?