My buddy has a 92 si and when he got it the speedo wouldn't wok at all. He replaced the vss and now it only seems to read speed after the car warms up. What would cause this

 

I would check to make sure the VSS assembly is securely seated in the transmission. I have had an experience before where the mounting tab on the VSS assembly cracks and therefore there is nothing holding the VSS in position. The way the gears are cut on the bottom of the VSS assembly can produce an upward force pushing the VSS gear up and out of engagement. Before I realize this problem I would also get strange readings when everything was cold and then it would seat and start working...