Silence is a hard virtue. All the other virtues have the advantage that, when you practice them, people will praise you. Sometimes if your moral system is very different from your friends’ people will attack you for your virtues, but getting attacked by sufficiently horrible people can sometimes be just as gratifying as praise. But if you stay silent, there’s no praise and no attacks. By definition, no one even knows you made a courageous moral choice.
Due to map vs territory, I suspect that it's just fine to talk about virtue of silence, as long as one is silent about the things that need it.