OK if You Know
Vitter himself has confessed that he did indeed use the service and regretted his choices.
Limbaugh seemed to be arguing that Vitter is just human, subject to temptation like the rest of us. This shouldn't be seen as a reason to resign. Instead, what this situation shows is that Vitter knows right from wrong. Vitter admitted that what he did was wrong. While some might argue that Vitter's activity demonstrates that he is hypocritical to push for "family values" legislation while himself engaging the services of prostitutes, Limbaugh argued that Vitter's confession demonstrates that he has the truly conservative virtue of knowing what is right and what is wrong.
Thinking about this, I realized that what Limbaugh was saying is that when liberals find that their actions are in conflict with their values, they are inclined to change their values. When conservatives find that their actions are in conflict with their values, the only thing that really matters is their values. They are human so we can forgive them for not acting in accord with their values. We all do that. What we can't forgive is when people try to change our common values.
Somehow conservatives don't see the hypocrisy in saying that as long as you know what is right and what is wrong you can do whatever you feel like doing. We are human and humans make bad choices sometimes. Conservatives seem to think there's nothing hypocritical about that.