It means that the more attractive a vice is the more virtuous one would have to be to not indulge.
The harder it is to avoid a vice the more virtuous one is when avoiding it.
We of course strive to be virtuous, but when we are hypocritical - and thus indulge in vice - we show how hard it is to virtuous - the more we acknowledge the power of a vice - the more we are free to determine just how virtuous a virtue is.
I've reiterated it a few times - but all of these answers are not only missing it, but claiming it is something else.
As far as Hitchens goes, he was referencing it in connection to a concession that is made, in this quote hypocrisy is the concession the virtuous makes to a vice - which each time such a concession is made "compliments" such a virtue.