The phrase, "Allow me to vent," is erroneously used by some people who think it is a verbal wild card that allows them to rant and rave and then storm off or hang up. It is nothing more than rude and inappropriate behavior that has no place in the workplace. Since the individual who played this card is your boss's colleague, you are literally and figuratively in an awkward position when it comes to saying something to him.
You made the right move by telling your boss about this individual's behavior, but telling is not enough. Let your boss know you need his help in handling this situation. Tell him that when his colleague behaves like this, you suffer and your work suffers, too. If your boss does not offer to speak with the individual, ask him to do so. In addition, ask him what you should do if this individual goes into a venting mode in the future.
Most bosses do not appreciate having their assistants abused by anyone. There would be appropriate irony if your boss were to vent about this to his abusive colleague. | 
