The longer a discussion of veganism by non-vegans goes, the more likely it is for someone to mention that one vegan that one time who got so sick that she had to go back to eating meat and had a religious experience the first time she had butter.
This is what I was going to suggest! Every. damn. time!
Alternatively, "The longer the discussion about veganism by non-vegans, the greater the probability that the non-vegan party will mention say that they considered being vegan but just decided to eat 'happy meat' instead (bonus if said while eating meat of unknown / highly questionable origin) AND / OR cannot be vegan because he/she could never give up cheese."