The easy way it would be diagnosed as either a vehicle speed sensor vs a bad instrument panel cluster (GM has known issues with the IPC going bad) is with a scanner hooked up to read live data. The scanner would be looked at in the mph category and if it reads mph while you are moving and the speedo is not, then that is a bad IPC. If the mph reads -0- like the speedo the the VSS is bad. Someone will have to perform this simple test for you at a local shop or you could look at the specs and functions of scanners that are reasonably priced to see if they can read live data and in particular, MPH from the VSS. You could also see if there is an IPC code stored but again - special scanner.
Another tip is that if the VSS is the cause, it always affects the way the transmission shifts. Another tip is if the cruise control is inoperative and worked before then that would point to bad VSS. So, if shifting normal like always and cruise control works and you have speedo not working at the same time - bad IPC. GM has been replacing these on a campaign at a very reduced cost so call the local GM dealer if the IPC is bad because YOU will not be able to replace it as it needs to be programmed to work. The data in the old cluster is uploaded into the new one.