And of course, "Only Nixon could go to China" is not a Vulcan proverb any more than "Revenge is a dish best served cold" is a Klingon proverb. Nicholas Meyer seems to enjoy the joke of taking pre-existing real-life expressions and having characters attribute them to alien cultures.
http://en.wikipedia.org/wiki/Nixon_goes_to_China
http://en.wikipedia.org/wiki/Nixon_goes_to_China
The phrase originated prior to Nixon's actual visit to China. An early use of the phrase is found in a December 1971 U.S. News & World Report interview with then-United States Senate Democratic Leader Mike Mansfield, in a section summary lead that read "'Only a Nixon' Could Go to China". The actual quote from Mansfield (which he prefaces by noting he had heard it said before) was "Only a Republican, perhaps only a Nixon, could have made this break and gotten away with it."