To my knowledge there's no such option for vB, that doesn't also mis-represent the number of online users.
To expand: If, say, you made it so nobody ever goes Idle in the Shoutbox, and each time the Shoutbox refreshes it updates the "last activity" of a user, nobody would ever be shown as logged out, and could cause issues for your members such as not knowing whether their friends are AFK or ignoring them, etc.