What is the correct procedure in case of vertical stabilizer failure?
There is no procedure, generally a vertical stabilizer failure makes an airplane uncontrollable, meaning everyone dies. There have been examples where vertical stabilizers failed and people survived, see this question for details. A pilot local to me cut 2/3 of his vertical stabilizer on high tension power wires while scud running and still landed safely. 

When there's any control surface failure pilots will have to learn how to fly the airplane and control it using whatever means possible. Differential thrust and speed brakes have been used to control lateral movement (yaw), trim can be used if controls are seized, etc. 

Reducing speed may not be a good idea, or at least not all at once, if the vertical stabilizer fails airflow over whatever stump remains may be the only think keeping it straight. If the control is stuck then reducing speed would make sense, it all depends on the nature of the failure. Descending is absolutely a requirement, you have to land somewhere after all, but unless you've depressurized there's no need to rush it, you'd want to work the problem and find out what the new parameters are, then gingerly work your way down.