For a given virtue-ethics view, you can say "act as a virtuous person would act", or "act in a way that achieves the same consequences as a virtuous person."
This is precisely the circularity that I was talking about. Where do you get the substance from? How do you know which person is virtuous? Unlike "act as maximises average expected utility" (some form of consequentialism) or "don't do X" (primitive deontology), "act as a virtuous person would" is an empty statement.
For example, if somebody seems impulsive and thoughtless, I will judge them for that, even if I don't observe their impulsivity causing them to take actions with likely bad consequences.
Nobody says you look only at actual actions. You're concerned with actions that the person is predisposed to. The non-ethically-relevant action that you observe is still evidence that that person has a temperament that disposes them to ethically relevant and unfavourable actions.
"act as a virtuous person would" is an empty statement.
It's an empty statement until you tack on a concrete description of virtue. But that's not hard to do. Aristotle, for instance, gives a long discussion about bravery and prudence and wisdom and justice and so forth in the Ethics -- and he does it without having a full account of what makes an action good or bad.
I suspect that what you are viewing as vacuous is really an implicit appeal to widely shared and widely understood norms that determine what makes people admirable or blameworthy. ...
Disclaimer: I am not a philosopher, so this post will likely seem amateurish to the subject matter experts.
LW is big on consequentialism, utilitarianism and other quantifiable ethics one can potentially program into a computer to make it provably friendly. However, I posit that most of us intuitively use virtue ethics, and not deontology or consequentialism. In other words, when judging one's actions we intuitively value the person's motivations over the rules they follow or the consequences of said actions. We may reevaluate our judgment later, based on laws and/or actual or expected usefulness, but the initial impulse still remains, even if overridden. To quote Casimir de Montrond, "Mistrust first impulses; they are nearly always good" (the quote is usually misattributed to Talleyrand).
Some examples:
I am not sure how to classify religious fanaticism (or other bigotry), but it seems to require a heavy dose of virtue ethics (feeling righteous), in addition to following the (deontological) tenets of whichever belief, with some consequentialism (for the greater good) mixed in.
When I try to introspect my own moral decisions (like whether to tell the truth, or to cheat on a test, or to drive over the speed limit), I can usually find a grain of virtue ethics inside. It might be followed or overridden, sometimes habitually, but it is always there. Can you?