Or am I misinterpreting what you said?
I don't know what "the ultimate decision theory" is, but I suspect this decision theory will contain both consequentialist and virtue ethical elements. It will be "consequentialist" in the trivial sense of picking the best alternative. It will be "virtue ethical" in the sense that it will in general do different things depending on what it can infer about other players based on their source code. In this sense I don't think virtue ethics is a hack approximation to consequentialism, I think it is an orthogonal idea.
That said, I am still confused by what you are trying to say!
I think we are using different definitions. I think of ethics first as applied to choosing one's own decisions, and only second as a tool to analyze and predict the decisions of others.
If I were to program a decision bot, I would certainly employ a mixture of algorithms. Some of them would have a model of what it means to, say, be fair (a virtue) and generate possible actions based on that. Others would restrict possible actions based on a deontological rule such as "thou shalt not kill" (cf. Asimov's laws). Yet others would search the space of ...
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?