Assuming the evidence favors the false consensus effect, we may explain its relevance to the dispute about virtues by pointing out that, since people tend to make such rash inferences, they are prone to over-attributing traits. They could reason as follows: “Well, I helped these strange fellows advertise for Joe’s Bar, so almost anyone would do the same. I guess most people are helpful!” Such an inference, however, is at best dubious.
I do not see how the false consensus effect advances the argument.
Recently I summarized Joshua Greene's attempt to 'explain away' deontological ethics by revealing the cognitive algorithms that generate deontological judgments and showing that the causes of our deontological judgments are inconsistent with normative principles we would endorse.
Mark Alfano has recently done the same thing with virtue ethics (which generally requires a fairly robust theory of character trait possession) in his March 2011 article on the topic:
An overview of the 'situationist' attack on character trait possession can be found in Doris' book Lack of Character.