Size of vocabulary of highly educated Chinese person I've read in various places on the internet that the average vocabulary of a Chinese person is around 5,000 words.

How about highly educated Chinese people? E.g. those with PhD's or holding top positions in the government or corporations.

Estimating vocabulary size accurately is really difficult as far as I know. The average English speaker's vocabulary is estimated to be in the range of 10,000 to 30,000 words, but highly educated English speaker may possess a vocabulary of twice that. Of course estimates are highly variable depending on where you look and depending on what you count as a word.

So does a highly educated Chinese person know around 10,000 words?

Any references are appreciated.

Note that I don't mean to compare vocabulary size between English and Chinese here. Personally I think Chinese is likely a more efficient language on one hand as it may be that more information can often be compressed into fewer words. That would explain why an English speaker needs to know more words.
What differentiates Chinese characters from English words is that most Chinese characters are not only words but also morphemes. 

e.g.


  In the English word unlikely, there are three morphemes, un, like and
  ly, but only like is a word. 
  
  In this Chinese compound word 不可能, there are also three morphemes,
  不、可、能, but each of them constitutes a word.


The estimated 5000-word vocabulary is not an indicator of the number of words like 不可能 one masters, but of morphemes like 不、可、能. 

A senior high school student is requested to master at least 6600 common characters. A Ph.D. is likely to have a similar, if not higher, vocabulary, as however difficult the jargons he possesses are, these jargons can be translated into Chinese with common characters, for example, polychlorinated biphenyl is translated as 多氯联苯, which every junior high student should be able to recognize but perhaps not fully understand.